Document:

<PAGE>

                                                                   Exhibit 10.18

                                 MERCATA, INC.

                    AMENDED AND RESTATED CO-SALE AGREEMENT

     This Amended and Restated Co-Sale Agreement (the "Agreement") is made and
entered into as of this lst day of March, 2000, by and among Mercata, Inc., a
Delaware corporation (the "Company"), each of the persons and entities listed on
Exhibit A hereto (the "Investors"), and Vulcan Ventures Incorporated (referred
to herein as the "Founder").

                                   Recitals

     Whereas, the Founder is the beneficial owner of an aggregate of one million
(1,000,000) shares of Common Stock of the Company, fourteen million (14,000,000)
shares of Series A Preferred Stock of the Company, one million eight hundred
eighty-eight thousand (1,888,000) shares of Series B Preferred Stock of the
Company and three million three hundred sixty-three thousand (3,363,000) shares
of Series C Preferred Stock of the Company;

     Whereas, the Company, the Founder and the Investors that have entered into
that certain Co-Sale Agreement, dated as of September 30, 1999, as amended (the
"Prior Co-Sale Agreement"), and desire to amend and restate such agreement with
the terms and conditions of this agreement;

     Whereas, Investors are purchasing shares of the Company's Series C
Preferred Stock (the "Series C Preferred Stock") pursuant to that certain Series
C Preferred Stock Purchase Agreement (the "Purchase Agreement") of even date
herewith;

     Whereas, such Investors were induced by the Company to purchase the Series
C Preferred Stock in part by the Company's and the Founder's agreement to enter
into this Agreement; and

     Whereas, the parties desire to enter into this Agreement in order to
grant rights of co-sale to each Investor.

     Now, Therefore, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth in this Agreement and the
Purchase Agreement and for other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, the parties agree hereto as
follows:

     1.   Definitions.

          (a)  "Co-Sale Stock" shall mean shares of the Company's Common Stock
and Preferred Stock now owned or subsequently acquired by the Founder by gift,
purchase, dividend, option exercise or any other means whether or not such
securities are only registered in a Founder's name or beneficially or legally
owned by Founder, including any interest of a spouse in any of the Co-Sale
Stock, whether that interest is asserted pursuant to marital property laws or
otherwise. The number of shares of Co-Sale Stock owned by the Founder as of the
date hereof are set forth on Exhibit B, which Exhibit may be amended from time
to time by the Company

                                       1.
<PAGE>

to reflect changes in the number of shares owned by the Founder, but the failure
to so amend shall have no effect on such Co-Sale Stock being subject to this
Agreement.

          (b)  "Common Stock" shall mean the Company's Common Stock and shares
of Common Stock issued or issuable upon exercise of any option, warrant or other
security or right of any kind convertible into or exchangeable for Common Stock.

          (c)  "Preferred Stock" shall mean the Company's Preferred Stock and
shares of Preferred Stock issued or issuable upon exercise of any option,
warrant or other security or right of any kind convertible into or exchangeable
for Preferred Stock.

          (d)  For the purpose of this Agreement, the term "Transfer" shall
include any sale, assignment, encumbrance, hypothecation, pledge, conveyance in
trust, gift, transfer by request, devise or descent, or other transfer or
disposition of any kind, including, but not limited to, transfers to receivers,
levying creditors, trustees or receivers in bankruptcy proceedings or general
assignees for the benefit of creditors, whether voluntary or by operation of
law, directly or indirectly, of any of the Co-Sale Stock.

          (e)  "Affiliate" of a person or entity means any person or entity who
controls, is controlled by or under common control with such person or entity.

     2.   Transfers by the Founder.

          (a)  If the Founder proposes to Transfer any shares of Co-Sale Stock
then the Founder shall promptly give written notice (the "Notice")
simultaneously to the Company and to each of the Investors at least thirty (30)
days prior to the closing of such Transfer. The Notice shall describe in
reasonable detail the proposed Transfer including, without limitation, the
number of shares of Co-Sale Stock to be transferred, the nature of such
Transfer, the consideration to be paid, and the name and address of each
prospective purchaser or transferee. In the event that the Transfer is being
made pursuant to the provisions of Section 3(a), the Notice shall state under
which section the Transfer is being made.

          (b)  Each Investor shall have the right, exercisable upon written
notice to the Founder within fifteen (15) days after the Notice, to participate
in such Transfer of Co-Sale Stock on the same terms and conditions. Such notice
shall indicate the number of shares of Common Stock and/or Preferred Stock such
Investor wishes to sell under his or her right to participate. To the extent one
or more of the Investors exercise such right of participation in accordance with
the terms and conditions set forth below, the number of shares of Co-Sale Stock
that the Founder may sell in the transaction shall be correspondingly reduced.

          (c)  Each Investor may sell all or any part of that number of shares
equal to the product obtained by multiplying (i) the aggregate number of shares
of Co-Sale Stock covered by the Notice by (ii) a fraction the numerator of which
is the number of shares of Common Stock (including shares of Preferred Stock on
an as converted basis) owned by such Investor at the time of the Transfer and
the denominator of which is the total number of shares of Common Stock
(including shares of Preferred Stock on an as converted basis) owned by Founder
and the Investors at the time of the Transfer.

                                       2.
<PAGE>

          (d)  Each Investor who elects to participate in the Transfer pursuant
to this Section 2 (a "Participant") shall effect its participation in the
Transfer by promptly delivering to Founder for transfer to the prospective
purchaser one or more certificates, properly endorsed for transfer, which
represent:

               (i)   the type and number of shares of Common Stock and/or
Preferred Stock which such Participant elects to sell; or

               (ii)  that number of shares of Preferred Stock which is at such
time convertible into the number of shares of Common Stock which such
Participant elects to sell; provided, however, that if the prospective purchaser
objects to the delivery of Preferred Stock in lieu of Common Stock, such
Participant shall convert such Preferred Stock into Common Stock and deliver
Common Stock as provided in Section 2(d)(i) above. The Company agrees to make
any such conversion concurrent with the actual transfer of such shares to the
purchaser.

          (e)  The stock certificate or certificates that the Participant
delivers to Founder pursuant to Section 2(d) shall be transferred to the
prospective purchaser in consummation of the sale of the Common Stock pursuant
to the terms and conditions specified in the Notice, and the Founder shall
concurrently therewith remit to such Participant that portion of the sale
proceeds to which such Participant is entitled by reason of its participation in
such sale. To the extent that any prospective purchaser or purchasers prohibits
such assignment or otherwise refuses to purchase shares or other securities from
a Participant exercising its rights of co-sale hereunder, the Founder shall not
sell to such prospective purchaser or purchasers any Co-Sale Stock unless and
until, simultaneously with such sale, the Founder shall purchase such shares or
other securities from such Participant on the same terms and conditions
specified in the Notice.

          (f)  The exercise or non-exercise of the rights of the Investors
hereunder to participate in one or more Transfers of Co-Sale Stock made by the
Founder shall not adversely affect their rights to participate in subsequent
Transfers of Co-Sale Stock subject to Section 2(a).

          (g)  If none of the Investors elect to participate in the sale of the
Co-Sale Stock subject to the Notice, the Founder may, not later than sixty (60)
days following delivery to the Company of the Notice, enter into an agreement
providing for the closing of the Transfer of the Co-Sale Stock covered by the
Notice within thirty (30) days of such agreement on terms and conditions not
more materially favorable to the transferor than those described in the Notice.
Any proposed transfer on terms and conditions materially more favorable than
those described in the Notice, as well as any subsequent proposed transfer of
any of the Co-Sale Stock by Founder, shall again be subject to the co-sale
rights of the Investors and shall require compliance by Founder with the
procedures described in this Section 2.

     3.   Exempt Transfers.

          (a)  Notwithstanding the foregoing, the co-sale rights of the
Investors shall not apply to (i) any transfer to the ancestors, descendants or
spouse or to trusts for the benefit of such persons or the Founder, (ii) any
transfer or transfers by a Founder to another Founder (the "Transferee-Founder")
so long as the Transferee-Founder is, at the time of the transfer, employed by
or acting as a consultant or director of the Company, (iii) any pledge of Co-
Sale

                                       3.
<PAGE>

Stock made pursuant to a bona fide loan transaction that creates a mere security
interest, (iv) any bona fide gift, or (v) any transfer to an Affiliate; provided
that in the event of any transfer made pursuant to one of the exemptions
provided above, (A) the Founder shall inform the Investors of such pledge,
transfer or gift prior to effecting it and (B) the pledgee, transferee or donee
shall furnish the Investors with a written agreement to be bound by and comply
with all provisions of Section 2. Except with respect to Co-Sale Stock
transferred under clause (i) above (which Co-Sale Stock shall no longer be
subject to the co-sale rights of the Investors), such transferred Co-Sale Stock
shall remain "Co-Sale Stock" hereunder, and such pledgee, transferee or donee
shall be treated as the "Founder" for purposes of this Agreement.

          (b)  Notwithstanding the foregoing, the provisions of Section 2 shall
not apply to the sale of any Co-Sale Stock to the public pursuant to a
registration statement filed with, and declared effective by, the Securities and
Exchange Commission under the Securities Act of 1933, as amended (the
"Securities Act").

          (c)  This Agreement is subject to, and shall in no manner limit the
right which the Company may have to repurchase securities from the Founder
pursuant to (i) a stock restriction agreement or other agreement between the
Company and the Founder and (ii) any right of first refusal set forth in the
Bylaws of the Company.

     4.   Prohibited Transfers.

          (a)  In the event that Founder should Transfer any Co-Sale Stock in
contravention of the co-sale rights of each Investor under this Agreement (a
"Prohibited Transfer"), each Investor, in addition to such other remedies as may
be available at law, in equity or hereunder, shall have the put option provided
below, and the Founder shall be bound by the applicable provisions of such
option.

          (b)  In the event of a Prohibited Transfer, each Investor shall have
the right to sell to Founder the type and number of shares of Common Stock equal
to the number of shares each Investor would have been entitled to transfer to
the purchaser under Section 2(c) hereof had the Prohibited Transfer been
effected pursuant to and in compliance with the terms hereof. Such sale shall be
made on the following terms and conditions:

               (i)   The price per share at which the shares are to be sold to
the Founder shall be equal to the price per share paid by the purchaser to the
Founder in such Prohibited Transfer. The Founder shall also reimburse each
Investor for any and all fees and expenses, including legal fees and expenses,
incurred pursuant to the exercise or the attempted exercise of the Investor's
rights under Section 2.

               (ii)  Within ninety (90) days after the date on which an Investor
received notice of the Prohibited Transfer or otherwise became aware of the
Prohibited Transfer, such Investor shall, if exercising the option created
hereby, deliver to the Founder the certificate or certificates representing
shares to be sold, each certificate to be properly endorsed for transfer.

               (iii) The Founder shall, upon receipt of the certificate or
certificates for the shares to be sold by an Investor, pursuant to this Section
4(b), pay the aggregate purchase

                                       4.
<PAGE>

price therefor and the amount of reimbursable fees and expenses, as specified in
Section 4(b)(i), in cash or by other means acceptable to the Investor.

               (iv)  Notwithstanding the foregoing, any attempt by a Founder to
transfer Co-Sale Stock in violation of Section 2 hereof shall be voidable at the
option of a majority in interest of the Investors if the Investors do not elect
to exercise the put option set forth in this Section 4, and the Company agrees
it will not effect such a transfer nor will it treat any alleged transferee as
the holder of such shares without the written consent of a majority in interest
of the Investors.

     5.   Legend.

          (a)  Each certificate representing shares of Co-Sale Stock now or
hereafter owned by the Founder or issued to any person in connection with a
transfer pursuant to Section 3(a) hereof shall be endorsed with the following
legend:

               "THE SALE, PLEDGE, HYPOTHECATION OR TRANSFER OF THE
     SECURITIES REPRESENTED BY THIS CERTIFICATE IS SUBJECT TO THE
     TERMS AND CONDITIONS OF A CERTAIN CO-SALE AGREEMENT BY AND
     BETWEEN THE SHAREHOLDER, THE COMPANY AND CERTAIN HOLDERS OF STOCK
     OF THE COMPANY. COPIES OF SUCH AGREEMENT MAY BE OBTAINED UPON
     WRITTEN REQUEST TO THE SECRETARY OF THE COMPANY."

          (b)  The Founder agrees that the Company may instruct its transfer
agent to impose transfer restrictions on the shares represented by certificates
bearing the legend referred to in Section 5(a) above to enforce the provisions
of this Agreement and the Company agrees to promptly do so. The legend shall be
removed upon termination of this Agreement.

     6.   Miscellaneous.

          (a)  Conditions to Exercise of Rights. Exercise of the Investors'
rights under this Agreement shall be subject to and conditioned upon, and the
Founder and the Company shall use their best efforts to assist each Investor in,
compliance with applicable laws.

          (b)  Governing Law. This Agreement shall be governed by and construed
under the laws of the State of Washington as applied to agreements among
Washington residents entered into and to be performed entirely within
Washington.

          (c)  Amendment. Any provision of this Agreement may be amended and the
observance thereof may be waived (either generally or in a particular instance
and either retroactively or prospectively), only by the written consent of (i)
as to the Company, only the Company, (ii) as to the Investors, persons holding
more than a majority in interest of the Common Stock (including shares of
Preferred Stock on an as converted basis) held by the Investors and their
assignees, pursuant to Section 6(d) hereof, and (iii) as to the Founder, only
the Founder; provided, that no consent of the Founder shall be necessary for any
amendment and/or restatement which includes additional holders of Preferred
Stock or other preferred stock of the Company as "Investors" and parties hereto.
Any amendment or waiver effected in

                                       5.
<PAGE>

accordance with clauses (i), (ii), and (iii) of this Section 6(c) shall be
binding upon each Investor, its successors and assigns, the Company and the
Founder.

          (d)  Assignment of Rights. This Agreement constitutes the entire
agreement between the parties relative to the specific subject matter hereof.
Any previous agreement among the parties relative to the specific subject matter
hereof is superseded by this Agreement. This Agreement and the rights and
obligations of the parties hereunder shall inure to the benefit of, and be
binding upon, their respective successors, assigns and legal representatives.
Each Investor may transfer or assign its rights hereunder only along with the
corresponding shares of Common Stock or Preferred Stock and any purported
transfer in violation of the foregoing shall be void and of no effect.

          (e)  Term. This Agreement shall continue in full force and effect from
the date hereof through the earliest of the following dates, on which date it
shall terminate in its entirety:

               (i)  the date of the closing of a firmly underwritten public
offering of the Common Stock pursuant to a registration statement filed with the
Securities and Exchange Commission, and declared effective under the Securities
Act of 1933, as amended;

               (ii) the date of the closing of a sale, lease, or other
disposition of all or substantially all of the Company's assets or the Company's
merger into or consolidation with any other corporation or other entity, or any
other corporate reorganization, in which the holders of the Company's
outstanding voting stock immediately prior to such transaction own, immediately
after such transaction, securities representing less than fifty percent (50%) of
the voting power of the corporation or other entity surviving such transaction,
provided, that this Section 6(e)(ii) shall not apply to a merger effected
exclusively for the purpose of changing the domicile of the Company; or

          (f)  the date as of which the parties hereto terminate this Agreement
by written consent of a majority in interest of the Investors (excluding Vulcan)
and a majority in interest of the Founder.

          (g)  Notices.  All notices required or permitted hereunder shall be in
writing and shall be deemed effectively given: (i) upon personal delivery to the
party to be notified, (ii) when sent by confirmed telex or facsimile if sent
during normal business hours of the recipient; if not, then on the next business
day, (iii) five (5) days after having been sent by registered or certified mail,
return receipt requested, postage prepaid, or (iv) one (1) day after deposit
with a nationally recognized overnight courier, specifying next day delivery,
with written verification of receipt.  All communications shall be sent to the
party to be notified at the address as set forth on the signature page hereof or
at such other address as such party may designate by ten (10) days advance
written notice to the other parties hereto.

          (h)  Severability.  In the event one or more of the provisions of this
Agreement should, for any reason, be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality, or unenforceability
shall not affect any other provisions of this

                                       6.
<PAGE>

Agreement, and this Agreement shall be construed as if such invalid, illegal or
unenforceable provision had never been contained herein.

          (i)  Attorneys' Fees. In the event that any suit or action is
instituted to enforce any provision in this Agreement, the prevailing party in
such dispute shall be entitled to recover from the losing party all fees, costs
and expenses of enforcing any right of such prevailing party under or with
respect to this Agreement, including without limitation, such reasonable fees
and expenses of attorneys and accountants, which shall include, without
limitation, all fees, costs and expenses of appeals.

          (j)  Entire Agreement. This Agreement and the Exhibits hereto, along
with the Purchase Agreement and each of the Exhibits thereto, constitute the
full and entire understanding and agreement between the parties with regard to
the subjects hereof and thereof and no party shall be liable or bound to any
other in any manner by any representations, warranties, covenants and agreements
except as specifically set forth herein and therein.

          (k)  Additional Investors.  Notwithstanding anything to the contrary
contained herein, if the Company shall issue additional shares of its Series C
Preferred Stock pursuant to the Purchase Agreement, any purchaser of such shares
of Series C Preferred Stock may become a party to this Agreement by executing
and delivering an additional counterpart signature page to this Agreement and
shall be deemed an "Investor" hereunder.

          (l)  Counterparts.  This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

          (m)  Amendment of Prior Co-Sale Agreement. The Prior Co-Sale Agreement
is hereby amended in its entirety herein. Such amendment and restatement is
effective upon the execution of this Agreement by the Company, the Founders and
the holders of a majority in interest of the Common Stock (on an as converted
basis) held by the Investors and their assigns outstanding as of the date of
this Agreement. Upon such execution, all provisions of, rights granted and
covenants made in the Prior Co-Sale Agreement are hereby waived, released and
amended in their entirety and shall have no further force or effect.

                     [THIS SPACE INTENTIONALLY LEFT BLANK]

                                       7.
<PAGE>

     The foregoing Amended and Restated Co-Sale Agreement is hereby executed as
of the date first above written.

                              COMPANY:

                              Mercata, Inc.

                              By: /s/ Tom Van Horn
                                 --------------------------------
                                  Tom Van Horn
                                  President

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              FOUNDER:

                              Vulcan Ventures Incorporated

                              By: /s/ William D. Savoy
                                 -------------------------------------
                                  William D. Savoy
                                  Vice President

                              INVESTORS:

                                /s/ William D. Savoy
                              ----------------------------------------
                              William D. Savoy

                                /s/ Bert Kolde
                              ----------------------------------------
                              Bert Kolde

                                /s/ Diane Daggatt
                              ----------------------------------------
                              Diane Daggatt

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTORS(S):

                              Highland Capital Partners IV Limited Partnership
                                   By: Highland Management Partners IV LLC
                                       Its:  General Partner

                                  By: /s/ Daniel Nova
                                     -----------------------------------
                                      Member

                              Highland Entrepreneurs' Fund IV Limited
                              Partnership
                                   By: Highland Management Partners IV LLC
                                       Its:  General Partner

                                  By: /s/ Daniel Nova
                                     -----------------------------------
                                      Member

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTORS(S):

                              Amerindo Technology Growth Fund II, Inc.

                              By:  /s/ Gary A. Tanaka
                                  --------------------------------------
                              Name:   Gary A. Tanaka
                                    ------------------------------------
                              Title:  Director
                                     -----------------------------------

                              Vertex Capital II LLC

                              By:  /s/ [ILLEGIBLE]
                                  --------------------------------------
                              Name: ------------------------------------
                              Title: -----------------------------------

                              Vertex Capital III LLC

                              By:  /s/ [ILLEGIBLE]
                                  --------------------------------------
                              Name:-------------------------------------
                              Title:------------------------------------

                              Sands Brothers/Amerindo Technology Associates LLC

                              By:  /s/ Martin S. Sands
                                  --------------------------------------
                              Name:  Martin S. Sands
                                    ------------------------------------
                              Title:  Manager
                                     -----------------------------------

                              Sands Brothers/Technology Associates Institution
                              LLC

                              By:  /s/ Martin S. Sands
                                  --------------------------------------
                              Name:  Martin S. Sands
                                    ------------------------------------
                              Title:  Manager
                                     -----------------------------------

                              Sands Brothers/Amerindo Technology Offshore
                              Associates LLC

                              By:  /s/ Martin S. Sands
                                  --------------------------------------
                              Name:  Martin S. Sands
                                    ------------------------------------
                              Title:  Manager
                                     -----------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              Litton Master Trust

                              By:  /s/ Joaquin Garcia-Larrieu
                                  --------------------------------------
                              Name:  Joaquin Garcia-Larrieu
                                    ------------------------------------
                              Title: Attorney-in-Fact
                                    ------------------------------------

                                /s/ James Stableford
                              ------------------------------------------
                              James Stableford

                              Aurora Technology Fund LLC

                              By:   /s/ [ILLEGIBLE]
                                  --------------------------------------
                              Name: ------------------------------------
                              Title: -----------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTORS(S):

                              Beagle Limited

                              By: ______________________________________
                              Name: ____________________________________
                              Title: ___________________________________

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Global Retail Partners, L.P.
                                   By: Global Retail Partners, Inc.
                                       Its:  General Partner

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                              Dlj Diversified Partners, L.P.
                                   By: DLJ Diversified Partners, Inc.
                                       Its:   General Partner

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                              Dlj Diversified Partners-A, L.P.
                                   By: DLJ Diversified Partners, Inc.
                                       Its:   General Partner

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                              GRP Partners, L.P.
                                   By: Global Retail Partners, Inc.
                                       Its:  General Partner

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                              Global Retail Partners Funding, INC.

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                              DLJ ESC II, L.P.
                                   By: DLJ LBO Plans Management Corporation
                                       Its:   General Partner

                              By:  /s/ Osamu R. Watanabe
                                  --------------------------------------
                              Name:  Osamu R. Watanabe
                                    ------------------------------------
                              Title:  Vice President
                                     -----------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Waelinvest

                              By:  /s/ Freddy De Greef
                                  --------------------------------------
                              Name:  Freddy De Greef
                                    ------------------------------------
                              Title: Chief Executive Officer
                                     -----------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Watershed Capital I, L.P.

                                   By:  Watershed Capital G.P. I, L.P.
                                        Its General Partner

                                        By:  Watershed Capital G.P. I, L.L.C.
                                             Its General Partner

                              By: __________________________________________
                                  Ralph C. Derrickson
                                  Managing Member

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                              INVESTOR(S):

                              TWP Mercata Investors

                              By: /s/ David Baylor
                                 --------------------------------
                                 David A. Baylor
                                 Managing Partner

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               CO-SALE AGREEMENT
<PAGE>

                                   Exhibit A

                               LIST OF INVESTORS

Highland Capital Partners IV Limited Partnership

Highland Entrepreneurs' Fund IV Limited Partnership

Amerindo Technology Growth Fund II, Inc. (referred to as "ATGF II in the
                                          Prior Co-Sale Agreement)

Vertex Capital II LLC

Vertex Capital III LLC

Sands Brothers/Amerindo Technology Associates LLC

Sands Brothers/Amerindo Technology Associates Institution LLC

Sands Brothers/Amerindo Technology Offshore Associates LLC

Litton Master Trust

James Stableford

Aurora Technology Fund LLC

Beagle Limited

Global Retail Partners, L.P.

DLJ Diversified Partners, L.P.

DLJ Diversified Partners - A, L.P.

GRP Partners, L.P.

Global Retail Partners Funding, Inc.

DLJ ESC II L.P.

Waelinvest

Watershed Capital I, L.P.

William D. Savoy

Diane H. Daggatt

Bert Kolde

TWP Mercata Investors

                         EXHIBIT A - LIST OF INVESTORS
<PAGE>

                                   Exhibit B

                            CO-SALE STOCK OWNERSHIP

Name of Founder                            Co-Sale Stock

Vulcan Ventures Incorporated        1,000,000 shares of Common Stock
                                   14,000,000 shares of Series A Preferred Stock
                                    1,888,000 shares of Series B Preferred Stock
                                      573,145 shares of Series C Preferred Stock

                      EXHIBIT B - CO-SALE STOCK OWNERSHIP<PAGE>

                                                                   Exhibit 10.19

                                 MERCATA, INC.

                     AMENDED AND RESTATED VOTING AGREEMENT

     This Amended and Restated Voting Agreement (this "Agreement") is made and
entered into as of this lst day of March, 2000, by and among Mercata, Inc., a
Delaware corporation (the "Company"), the persons and entities listed on Exhibit
A hereto (the "Investors").

                                  Witnesseth

     Whereas, the Investors are purchasing shares of the Company's Series C
Preferred Stock (the "Series C Preferred Stock"), pursuant to that certain
Series C Preferred Stock Purchase Agreement (the "Purchase Agreement") of even
date herewith (the "Financing");

     Whereas, the Company and the Investors have entered into that certain
Voting Agreement, dated as of September 30, 1999, as amended (the "Prior Voting
Agreement"), and desire to amend and restate such agreement with the terms and
conditions of this agreement; and

     Whereas, in connection with the consummation of the Financing, the Company
and the Investors have agreed to provide for the future voting of their shares
of the Company's capital stock as set forth below.

     Now, Therefore, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

                                   Agreement

1.   Voting

     1.1     Investor Shares.  The Investors each agree to hold all shares of
voting capital stock of the Company registered in their respective names or
beneficially owned by them as of the date hereof and any and all other
securities of the Company legally or beneficially acquired by each of the
Investors after the date hereof (hereinafter collectively referred to as the
"Investor Shares") subject to, and to vote the Investor Shares in accordance
with, the provisions of this Agreement.

     1.2     Election of Directors. For so long as at least 10,000,000 shares of
Preferred Stock remain outstanding (subject to adjustment for stock splits,
dividends or the like) and the authorized size of the Company's Board of
Directors is seven (7) or more, on all matters relating to the election of
directors of the Company, the Investors agree to vote all Investor Shares held
by them (or the holders thereof shall consent pursuant to an action by written
consent of the holders of capital stock of the Company) so as to elect a member
of the Company's Board of Directors as follows:

                                       1.
<PAGE>

             (a)    At each election of directors in which the holders of Common
Stock and holders of Preferred Stock, voting together as a single class, are
entitled to elect directors of the Company, the Investors shall vote all of
their respective Investor Shares so as to elect: one (1) representative of
Global Retail Partners Funding, Inc. so long as it or its affiliates hold not
less than 1,792,000 shares of Preferred Stock (as adjusted for stock splits,
dividends and the like); provided, that in no event will this provision require
the Investors to vote any shares so as to elect more than one (1) representative
of Global Retail Partners Funding, Inc. to the Company's Board of Directors at
any one time. Any vote taken to remove any director elected pursuant to this
Section 1.2(a), or to fill any vacancy created by the resignation or death of a
director elected pursuant to this Section 1.2(a), shall also be subject to the
provisions of this Section 1.2(a).

             (b)    At each election of directors in which the holders of Series
A Preferred Stock, voting as a separate class, are entitled to elect directors
of the Company, the Investors shall vote all of their respective Investor Shares
so as to elect two (2) representatives of Vulcan Ventures Incorporated so long
as it holds not less than 11,200,000 shares of Series A Preferred Stock (as
adjusted for stock splits, dividends and the like). Any vote taken to remove any
director elected pursuant to this Section 1.2(b), or to fill any vacancy created
by the resignation or death of a director elected pursuant to this Section
1.2(b), shall also be subject to the provisions of this Section 1.2(b);

             (c)    At each election of directors in which the holders of Series
B Preferred Stock, voting as a separate class, are entitled to elect directors
of the Company, the Investors shall vote all of their respective Investor Shares
so as to elect one designee of Highland Capital Partners IV Limited Partnership
so long as it holds not less than 4,480,000 shares of Series B Preferred Stock
(as adjusted for stock splits, dividends and the like). Any vote taken to remove
any director elected pursuant to this Section 1.2(c), or to fill any vacancy
created by the resignation or death of a director elected pursuant to this
Section 1.2(c), shall also be subject to the provisions of this Section 1.2(c).

             (d)    At each election of directors in which the holders of Common
Stock, voting as a separate class, are entitled to elect directors of the
Company, the Investors shall vote all of their respective Investor Shares so as
to elect: (i) the person serving as Chief Executive Officer of the Company, or
if there is no duly elected Chief Executive Officer, one (1) individual
nominated by the holders of a majority in interest of the Common Stock and (ii)
one (1) individual nominated by the holders of a majority in interest of the
Common Stock. Any vote taken to remove any director elected pursuant to this
Section 1.2(d), or to fill any vacancy created by the resignation or death of a
director elected pursuant to this Section 1.2(d), shall also be subject to the
provisions of this Section 1.2(d).

     1.3     Legend

             (a)    Concurrently with the execution of this Agreement, there
shall be imprinted or otherwise placed, on certificates representing the
Investor Shares the following restrictive legend (the "Legend"):

                                       2.
<PAGE>

          "THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO
     THE TERMS AND CONDITIONS OF A VOTING AGREEMENT WHICH PLACES
     CERTAIN RESTRICTIONS ON THE VOTING OF THE SHARES REPRESENTED
     HEREBY. ANY PERSON ACCEPTING ANY INTEREST IN SUCH SHARES SHALL BE
     DEEMED TO AGREE TO AND SHALL BECOME BOUND BY ALL THE PROVISIONS
     OF SUCH AGREEMENT. A COPY OF SUCH VOTING AGREEMENT WILL BE
     FURNISHED TO THE RECORD HOLDER OF THIS CERTIFICATE WITHOUT CHARGE
     UPON WRITTEN REQUEST TO THE COMPANY AT ITS PRINCIPAL PLACE OF
     BUSINESS."

             (b)    The Company agrees that, during the term of this Agreement,
it will not remove, and will not permit to be removed (upon registration of
transfer, reissuance of otherwise), the Legend from any such certificate and
will place or cause to be placed the Legend on any new certificate issued to
represent Investor Shares theretofore represented by a certificate carrying the
Legend.

     1.4     Successors.  The provisions of this Agreement shall be binding upon
the successors in interest to any of the Investor Shares.  The Company shall not
permit the transfer of any of the Investor Shares on its books or issue a new
certificate representing any of the Investor Shares unless and until the person
to whom such security is to be transferred shall have executed a written
agreement, substantially in the form of this Agreement, pursuant to which such
person becomes a party to this Agreement and agrees to be bound by all the
provisions hereof as if such person were an Investor.

     1.5     Other Rights.  Except as provided by this Agreement or any other
agreement entered into in connection with the Financing, each Investor shall
exercise the full rights of a holder of capital stock of the Company with
respect to the Investor Shares.

2.   Termination

     2.1     This Agreement shall continue in full force and effect from the
date hereof through the earliest of the following dates, on which date it shall
terminate in its entirety:

             (a)    the date of the closing of a firmly underwritten public
offering of the Common Stock pursuant to a registration statement filed with the
Securities and Exchange Commission, and declared effective under the Securities
Act of 1933, as amended;

             (b)    at such time as less than 10,000,000 shares of Preferred
Stock remain outstanding (as adjusted for stock splits, dividends or the like);

             (c)    ten (10) years from the date of this Agreement;

             (d)    the date of the closing of a sale, lease, or other
disposition of all or substantially all of the Company's assets or the Company's
merger into or consolidation with any other corporation or other entity, or any
other corporate reorganization, in which the holders of the Company's
outstanding voting stock immediately prior to such transaction own, immediately
after such transaction, securities representing less than fifty percent (50%) of
the

                                       3.
<PAGE>

voting power of the corporation or other entity surviving such transaction,
provided that this Section 2.1(d) shall not apply to a merger effected
exclusively for the purpose of changing the domicile of the Company; or

             (e)    the date as of which the parties hereto terminate this
Agreement by written consent of the holders of seventy percent (70%) in interest
of the Investors.

3.   Miscellaneous

     3.1     Specific Performance.  The parties hereto hereby declare that it is
impossible to measure in money the damages which will accrue to a party hereto
or to their heirs, personal representatives, or assigns by reason of a failure
to perform any of the obligations under this Agreement and agree that the terms
of this Agreement shall be specifically enforceable.  If any party hereto or his
heirs, personal representatives, or assigns institutes any action or proceeding
to specifically enforce the provisions hereof, any person against whom such
action or proceeding is brought hereby waives the claim or defense therein that
such party or such personal representative has an adequate remedy at law, and
such person shall not offer in any such action or proceeding the claim or
defense that such remedy at law exists.

     3.2     Governing Law.  This Agreement, and the rights of the parties
hereto, shall be governed by and construed in accordance with the laws of the
State of Washington as such laws apply to agreements among Washington residents
made and to be performed entirely within the State of Washington.

     3.3     Amendment or Waiver.  This Agreement may be amended (or provisions
of this Agreement waived) only by an instrument in writing signed by (a) the
Company and (b) seventy percent (70%) in interest of the Investors. Any
amendment or waiver so effected shall be binding upon the Company, each of the
parties hereto and any assignee of any such party.

     3.4     Severability.  In the event one or more of the provisions of this
Agreement should, for any reason, be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provisions of this Agreement, and this Agreement
shall be construed as if such invalid, illegal or unenforceable provision had
never been contained herein.

     3.5     Successors.  This Agreement shall inure to the benefit of and be
binding upon the parties hereto and their respective heirs, successors, assigns,
administrators, executors and other legal representatives.

     3.6     Additional Shares.  In the event that subsequent to the date of
this Agreement any shares or other securities are issued on, or in exchange for,
any of the Investor Shares by reason of any stock dividend, stock split,
combination of shares, reclassification or the like, such shares or securities
shall be deemed to be Investor Shares for purposes of this Agreement.

     3.7     Addition of Investors.  Notwithstanding anything to the contrary
contained herein, if the Company shall issue additional shares of its Preferred
Stock pursuant to the Purchase Agreement, any purchaser of such shares of
Preferred Stock may become a party to this

                                       4.
<PAGE>

Agreement by executing and delivering an additional counterpart signature page
to this Agreement and shall be deemed an "Investor" hereunder.

     3.8     Counterparts.  This Agreement may be executed in one or more
counterparts, each of which will be deemed an original but all of which together
shall constitute one and the same agreement.

     3.9     Waiver.  No waivers of any breach of this Agreement extended by any
party hereto to any other party shall be construed as a waiver of any rights or
remedies of any other party hereto or with respect to any subsequent breach.

     3.10    Attorneys' Fees.  In the event that any suit or action is
instituted to enforce any provision in this Agreement, the prevailing party in
such dispute shall be entitled to recover from the losing party all fees, costs
and expenses of enforcing any right of such prevailing party under or with
respect to this Agreement, including without limitation, such reasonable fees
and expenses of attorneys and accountants, which shall include, without
limitation, all fees, costs and expenses of appeals.

     3.11    Notices.  Any notices required in connection with this Agreement
shall be in writing and shall be deemed effectively given: (a) upon personal
delivery to the party to be notified, (b) when sent by confirmed facsimile if
sent during normal business hours of the recipient; if not, then on the next
business day, (c) five (5) days after having been sent by registered or
certified mail, return receipt requested, postage prepaid, or (d) one (1) day
after deposit with a nationally recognized overnight courier, specifying next
day delivery, with written notification of receipt. All notices shall be
addressed to the holder appearing on the books of the Company or at such address
as such party may designate by ten (10) days advance written notice to the other
parties hereto.

     3.12    Entire Agreement.  This Agreement and the Exhibits hereto, along
with the Purchase Agreement and each of the Exhibits thereto, constitute the
full and entire understanding and agreement between the parties with regard to
the subjects hereof and thereof and no party shall be liable or bound to any
other in any manner by any representations, warranties, covenants and agreements
except as specifically set forth herein and therein.

     3.13    Amendment of Prior Voting Agreement.  The Prior Voting Agreement is
hereby amended in its entirety and restated herein.  Such amendment and
restatement is effective upon the execution of this Agreement by the Company,
seventy percent (70%) in interest of the "Investors" (as defined in the Prior
Voting Agreement).  Upon such execution, all provisions of rights granted and
covenants made in the Prior Voting Agreement are hereby waived, released and
terminated in their entirety and shall have no further force or effect.

                     [This Space Intentionally Left Blank]

                                       5.
<PAGE>

     In Witness Whereof, the parties hereto have executed this Amended and
Restated Voting Agreement as of the date first above written to become effective
as of the date set forth in the first paragraph hereof.

                                        COMPANY:

                                        Mercata, Inc.

                                        By: /s/ Tom Van Horn
                                           ---------------------------
                                           Tom Van Horn
                                           President

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                                        INVESTOR(S):

                                        Vulcan Ventures Incorporated

                                        By: /s/ William  D. Savoy
                                           -------------------------------
                                            William D. Savoy
                                            Vice President

                                        /s/ William D. Savoy
                                        ----------------------------------
                                        William D. Savoy

                                        /s/ Bert Kolde
                                        ----------------------------------
                                        Bert Kolde

                                        /s/ Diane Daggatt
                                        ----------------------------------
                                        Diane Daggatt

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                                        INVESTOR(S):

                                        Highland Capital Partners IV Limited
                                        Partnership

                                         By: Highland Management Partners IV LLC
                                             Its:  General Partner

                                             By:/s/ Daniel Nova
                                                -------------------------------
                                                Member

                                        Highland Entrepreneurs' Fund IV Limited
                                        Partnership

                                         By: Highland Management Partners IV LLC
                                             Its:  General Partner

                                             By:/s/ Daniel Nova
                                                -------------------------------
                                                Member

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Amerindo Technology Growth Fund II, Inc.

                              By:  /s/ Gary A. Tanaka
                                 -----------------------------------
                              Name:    Gary A. Tanaka
                                   ---------------------------------
                              Title:    Director
                                    --------------------------------

                              Vertex Capital II LLC

                              By:  /s/ [ILLEGIBLE]
                              Name:_________________________________
                              Title:________________________________

                              Vertex Capital III LLC

                              By:  /s/ [ILLEGIBLE]
                              Name:_________________________________
                              Title:________________________________

                              Sands Brothers/Amerindo Technology Associates LLC

                              By:  /s/ Martin S. Sands
                                 -----------------------------------
                              Name:    Martin S. Sands
                                   ---------------------------------
                              Title:   Manager
                                    --------------------------------

                              Sands Brothers/Technology Associates Institution
                              LLC

                              By:  /s/ Martin S. Sands
                                 -----------------------------------
                              Name:    Martin S. Sands
                                   ---------------------------------
                              Title:   Manager
                                    --------------------------------

                              Sands Brothers/Amerindo Technology Offshore
                              Associates LLC

                              By:  /s/ Martin S. Sands
                                 -----------------------------------
                              Name:    Martin S. Sands
                                   ---------------------------------
                              Title:   Manager
                                    --------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              Litton Master Trust

                              By: /s/ Joaquin Garcia-Larrieu
                                 -----------------------------------
                              Name: Joaquin Garcia-Larrieu
                                   ---------------------------------
                              Title: Attorney-in-Fact
                                    --------------------------------

                              /s/ James Stableford
                              --------------------------------------
                              James Stableford

                              Aurora Technology Fund LLC

                              By: /s/ [ILLEGIBLE]
                                 -----------------------------------
                              Name:_________________________________
                              Title:________________________________

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Beagle Limited

                              By:___________________________________

                              Name:_________________________________

                              Title:________________________________

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Global Retail Partners, L.P.
                                 By: Global Retail Partners, Inc.
                                   Its:  General Partner

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                              DLJ Diversified Partners, L.P.

                                 By: DLJ Diversified Partners, Inc.
                                 Its:   General Partner

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                              DLJ Diversified Partners-A, L.P.
                                 By: DLJ Diversified Partners, Inc.
                                   Its: General Partner

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                              GRP Partners, L.P.
                                 By: Global Retail Partners, Inc.
                                   Its: General Partner

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                              Global Retail Partners Funding, Inc.

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                              DLJ ESC II, L.P.
                                 By: DLJ LBO Plans Management Corporation
                                 Its:   General Partner

                                 By:  /s/ Osamu R. Watanabe
                                     ----------------------------------
                                 Name: Osamu R. Watanabe
                                      ---------------------------------
                                 Title: Vice President
                                       --------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Waelinvest

                              By: /s/ Freddy De Greef
                                 --------------------------------------
                              Name: Freddy De Greef
                                   ------------------------------------
                              Title: Chief Executive Officer
                                    -----------------------------------

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              Watershed Capital I, L.P.

                              By: Watershed Capital G.P. I, L.P.
                                  Its General Partner

                                  By: Watershed Capital G.P. I, L.L.C.
                                      Its General Partner

                                  By:______________________________________
                                     Ralph C. Derrickson
                                     Managing Member

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                              INVESTOR(S):

                              TWP Mercata Investors

                              By: /s/ David Baylor
                                 -----------------------------------
                                 David A. Baylor
                                 Managing Partner

                    SIGNATURE PAGE TO AMENDED AND RESTATED
                               VOTING AGREEMENT
<PAGE>

                                   Exhibit A

                               LIST OF INVESTORS

Investors

Vulcan Ventures Incorporated

Highland Capital Partners IV Limited Partnership

Highland Entrepreneurs' Fund IV Limited Partnership

Amerindo Technology Growth Fund II, Inc. (referred to as "ATGF II" in the
                                            Prior Voting Agreement")

Vertex Capital II LLC

Vertex Capital III LLC

Sands Brothers/Amerindo Technology Associates LLC

Sands Brothers/Amerindo Technology Associates Institution LLC

Sands Brothers/Amerindo Technology Offshore Associates LLC

Litton Master Trust

James Stableford

Aurora Technology Fund LLC

Beagle Limited

Global Retail Partners, L.P.

DLJ Diversified Partners, L.P.

DLJ Diversified Partners - A, L.P.

GRP Partners, L.P.

Global Retail Partners Funding, Inc.

DLJ ESC II L.P.

Waelinvest

Watershed Capital I, L.P.

William D. Savoy

Diane H. Daggatt

Bert Kolde

TWP Mercata Investors

                         EXHIBIT A - LIST OF INVESTORS

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