<PAGE>

                                                                   EXHIBIT 10.11

                                 MERCATA, INC.

                  SERIES A PREFERRED STOCK PURCHASE AGREEMENT

<PAGE>

                               Table of Contents

<TABLE>
<CAPTION>                                                                  Page
<S>                                                                        <C>
1.   Agreement To Sell And Purchase.......................................    1
     1.1  Authorization of Shares.........................................    1
     1.2  Sale and Purchase...............................................    1

2.   Closing, Delivery And Payment........................................    2
     2.1  Closing.........................................................    2
     2.2  Delivery........................................................    2

3.   Representations And Warranties Of The Company........................    2
     3.1  Organization, Good Standing and Qualification...................    2
     3.2  Capitalization; Voting Rights...................................    2
     3.3  Authorization; Binding Obligations..............................    3
     3.4  Offering Valid..................................................    3

4.   Representations And Warranties Of Purchaser..........................    3
     4.1  Investment Representations......................................    3
     4.2  Restrictions on Transfer........................................    3

5.   Miscellaneous........................................................    4
     5.1  Survival........................................................    4
     5.2  Entire Agreement................................................    4
     5.3  Severability....................................................    4
     5.4  Waiver of Conflicts.............................................    4
     5.5  Counterparts....................................................    4
     5.6  Broker's Fees...................................................    4
</TABLE>

                                      i.
<PAGE>

                               LIST OF EXHIBITS


Schedule of Purchasers                                Exhibit A

Restated Certificate                                  Exhibit B

                                      ii.
<PAGE>

                                                                   EXHIBIT 10.11

                                 MERCATA, INC.

                  SERIES A PREFERRED STOCK PURCHASE AGREEMENT

     This Series A Preferred Stock Purchase Agreement (the "Agreement") is
entered into as of March 5, 1999, by and among Mercata, Inc., a Delaware
corporation (the "Company") and the entity whose names are set forth on the
Schedule of Purchasers attached hereto as Exhibit A ("Purchaser").

                                   Recitals

     Whereas, the Company has authorized the sale and issuance of an
aggregate of fourteen million (14, 000,000) shares of its Series A Preferred
Stock (the "Shares");

     Whereas, Purchaser desire to purchase the Shares on the terms and
conditions set forth herein; and

     Whereas, the Company desires to issue and sell the Shares to Purchaser
on the terms and conditions set forth herein;

     Now, Therefore, in consideration of the foregoing recitals and the
mutual promises hereinafter set forth, the parties hereto agree as follows:

     1.   Agreement To Sell And Purchase.

          1.1  Authorization of Shares.  On or prior to the Closing (as defined
in Section 2 below), the Company shall have authorized (a) the sale and issuance
to Purchaser of the Shares and (b) the issuance of such shares of Common Stock
to be issued upon conversion of the Shares (the "Conversion Shares").  The
Shares and the Conversion Shares shall have the rights, preferences, privileges
and restrictions set forth in the Amended and Restated Certificate of
Incorporation of the Company, in the form attached hereto as Exhibit B (the
"Restated Charter").

          1.2  Sale and Purchase.  Subject to the terms and conditions hereof,
at the Closing (as hereinafter defined) the Company hereby agrees to issue and
sell to Purchaser and Purchaser agrees to purchase from the Company the number
of Shares set forth opposite Purchaser's name on Exhibit A, at a purchase price
of one dollar and five cents ($1.05) per share.

                                       1.
<PAGE>

     2.   Closing, Delivery And Payment.

          2.1  Closing.  The closing of the sale and purchase of the Shares
under this Agreement (the "Closing") shall take place at 5:00 p.m. on the date
hereof, at the offices of Cooley Godward llp, 4205 Carillon Point, Kirkland,
Washington 98033-7355 or at such other time or place as the Company and
Purchaser may mutually agree (such date is hereinafter referred to as the
"Closing Date").

          2.2  Delivery.  At the Closing, subject to the terms and conditions
hereof, the Company will deliver to Purchaser a certificate representing the
number of Shares to be purchased at the Closing by Purchaser, against payment of
the purchase price therefor by check, wire transfer made payable to the order of
the Company, cancellation of indebtedness or any combination of the foregoing.

     3.   Representations And Warranties Of The Company.

          Except as set forth on a Schedule of Exceptions delivered by the
Company to  Purchaser at the Closing, the Company hereby represents and warrants
to Purchaser as of the date of this Agreement as follows:

          3.1  Organization, Good Standing and Qualification.  The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware.  The Company has all requisite corporate power and
authority to own and operate its properties and assets, to execute and deliver
this Agreement, to issue and sell the Shares and the Conversion Shares, and to
carry out the provisions of this Agreement and the Restated Charter and to carry
on its business as presently conducted and as presently proposed to be
conducted.

          3.2  Capitalization; Voting Rights.  The authorized capital stock of
the Company, immediately prior to the Closing, will consist of twenty-one
million (21,000,000) shares of Common Stock, 1,000,000 shares of which are
issued and outstanding and five million (5,000,000) shares of which are reserved
for future issuance pursuant to the Company's 1999 Equity Incentive Plan, and
fourteen million (14,000,000) shares of Preferred Stock, all of which are
designated Series A Preferred Stock, none of which are issued and outstanding.
All issued and outstanding shares of the Company's Common Stock (a) have been
duly authorized and validly issued, and (b) are fully paid and nonassessable.
The rights, preferences, privileges and restrictions of the Shares are as stated
in the Restated Charter.  Each series of Preferred Stock is convertible into
Common Stock on a one-for-one basis.  The Conversion Shares have been duly and
validly reserved for issuance.  Other than the five million (5,000,000) shares
reserved for issuance under the Company's 1999 Equity Incentive Plan, there are
no outstanding options, warrants, rights (including conversion or preemptive
rights and rights of first refusal), proxy or shareholder agreements, or
agreements of any kind for the purchase or acquisition from the Company of any
of its securities.  When issued in compliance with the provisions of this
Agreement and the Restated Charter, the Shares and the Conversion Shares will be
validly issued, fully paid and nonassessable, and will be free of any liens or
encumbrances other than liens and encumbrances created by or imposed upon
Purchaser; provided, however, that the Shares and the Conversion Shares may be
subject to restrictions on transfer under state and/or

                                       2.
<PAGE>

federal securities laws as set forth herein or as otherwise required by such
laws at the time a transfer is proposed.

          3.3  Authorization; Binding Obligations.  All corporate action on the
part of the Company, its officers, directors and shareholders necessary for the
authorization of this Agreement, the performance of all obligations of the
Company hereunder at the Closing and the authorization, sale, issuance and
delivery of the Shares pursuant hereto and the Conversion Shares pursuant to the
Restated Charter has been taken or will be taken prior to the Closing.  The
Agreement, when executed and delivered, will be valid and binding obligations of
the Company enforceable in accordance with their terms, except (a) as limited by
applicable bankruptcy, insolvency, reorganization, moratorium or other laws of
general application affecting enforcement of creditors' rights, and (b) general
principles of equity that restrict the availability of equitable remedies.  The
sale of the Shares and the subsequent conversion of the Shares into Conversion
Shares are not and will not be subject to any preemptive rights or rights of
first refusal that have not been properly waived or complied with.

          3.4  Offering Valid.  Assuming the accuracy of the representations and
warranties of Purchaser contained in Section 4.1 hereof, the offer, sale and
issuance of the Shares and the Conversion Shares will be exempt from the
registration requirements of the Securities Act of 1933, as amended (the
"Securities Act"), and will have been registered or qualified (or are exempt
from registration and qualification) under the registration, permit or
qualification requirements of all applicable state securities laws.  Neither the
Company nor any agent on its behalf has solicited or will solicit any offers to
sell or has offered to sell or will offer to sell all or any part of the Shares
to any person or persons so as to bring the sale of such Shares by the Company
within the registration provisions of the Securities Act or any state securities
laws.

     4.   Representations And Warranties Of Purchaser.

          4.1  Investment Representations.  Purchaser understands that the
Shares and Conversion Shares have not been registered under the Securities Act.
Purchaser also understands that the Shares are being offered and sold pursuant
to an exemption from registration contained in the Securities Act based in part
upon Purchaser's representations contained in the Agreement.  Purchaser hereby
represents and warrants that Purchaser is (a) acquiring the Shares for
Purchaser's own account for investment only, and not with a view towards their
distribution; and (b) an accredited investor within the meaning of Regulation D
under the Securities Act.

          4.2  Restrictions on Transfer.  Each certificate representing Shares
and Conversion Shares shall be stamped or otherwise imprinted with a legend
substantially similar to the following (in addition to any legend required under
applicable state securities laws):

     THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER
     THE SECURITIES ACT OF 1933 (THE "ACT") AND MAY NOT BE OFFERED,
     SOLD OR OTHERWISE TRANSFERRED, ASSIGNED, PLEDGED OR HYPOTHECATED
     UNLESS AND UNTIL REGISTERED UNDER THE ACT OR UNLESS THE COMPANY
     HAS

                                       3.
<PAGE>

     RECEIVED AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY AND
     ITS COUNSEL THAT SUCH REGISTRATION IS NOT REQUIRED.

     5.   Miscellaneous.

          5.1  Survival.  The representations, warranties, covenants and
agreements made herein shall survive any investigation made by Purchaser and the
closing of the transactions contemplated hereby.  All statements as to factual
matters contained in any certificate or other instrument delivered by or on
behalf of the Company pursuant hereto in connection with the transactions
contemplated hereby shall be deemed to be representations and warranties by the
Company hereunder solely as of the date of such certificate or instrument.

          5.2  Entire Agreement.  This Agreement, the Exhibits and Schedules
hereto, and the other documents delivered pursuant hereto constitute the full
and entire understanding and agreement between the parties with regard to the
subjects hereof 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.

          5.3  Severability.  In case any provision of the Agreement shall be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not in any way be affected or impaired thereby.

          5.4  Waiver of Conflicts.  Each party to this Agreement acknowledges
that legal counsel for the Company, Cooley Godward llp ("Cooley Godward"), has
in the past performed and may continue in the future to perform legal services
for Purchaser or their affiliates in matters unrelated to the transactions
contemplated by this Agreement, including, but not limited to, the
representation of Purchaser in matters of a similar nature to the transactions
contemplated herein.  Each party to this Agreement hereby (a) acknowledges that
they have had an opportunity to ask for and have obtained information relevant
to such representation, including disclosure of the reasonably foreseeable
adverse consequences of such representation; (b) acknowledges that with respect
to the transactions contemplated herein, Cooley Godward has represented the
Company and not Purchaser or any individual shareholder, director or employee of
the Company; and (c) gives its informed consent to Cooley Godward's
representation of the Company in the transactions contemplated by this Agreement
and Cooley Godward's previous or continuing representation of Purchaser or their
affiliates in matters unrelated to such transactions.

          5.5  Counterparts.  This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

          5.6  Broker's Fees.  Each party hereto represents and warrants that no
agent, broker, investment banker, person or firm acting on behalf of or under
the authority of such party hereto is or will be entitled to any broker's or
finder's fee or any other commission directly or indirectly in connection with
the transactions contemplated herein.  Each party hereto further

                                       4.
<PAGE>

agrees to indemnify each other party for any claims, losses or expenses incurred
by such other party as a result of the representation in this Section 6.14 being
untrue.

             [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                       5.
<PAGE>

     In Witness Whereof, the parties hereto have executed the Series A Preferred
Stock Purchase Agreement as of the date set forth in the first paragraph hereof.

Company:                               Purchaser:

Mercata, Inc.                          Vulcan Ventures Incorporated

By: /s/ Tom Van Horn                   By: /s/ [Illegible]
    -------------------------              ---------------------------
     President                         Title: Vice President
                                              ------------------------

                 SERIES A PREFERRED STOCK PURCHASE AGREEMENT
                                SIGNATURE PAGE

<PAGE>

                                   Exhibit A
                            SCHEDULE OF PURCHASERS


                                                            Aggregate
Name and Address                             Shares        Purchase Price

Vulcan Ventures Incorporated               14,000,000     $  14,700,000 1
110-110/th/ Avenue N.E., Suite 550
Bellevue, WA 98004

Total:                                     14,000,000     $  14,700,000
                                           ==========     =============


1 Includes cancellation of indebtedness of the Company under notes dated
October 13, 1998, December 22, 1998 and February 1, 1999 each in the principal
amounts of $1,200,000 and the aggregate principal amount of $3,600,000.  Also
includes contribution of 3,000,000 shares of Common Stock having a current fair
market value of ten cents ($0.10) per share and an aggregate fair market value
of three hundred thousand dollars ($300,000).  Each such note and all such
shares were originally issued to Paul G. Allen and subsequently assigned to
Vulcan Ventures Incorporated.  Vulcan Ventures Incorporated shall pay the
remaining ten million eight hundred thousand dollars ($10,800,000) of the
purchase price by check or wire transfer (less a credit of $_______________ to
reflect the cancellation of accrued but unpaid interest on the Notes owed by the
Company at the time of cancellation).
<PAGE>

                                   Exhibit B
                             RESTATED CERTIFICATE


                             AMENDED AND RESTATED

                         CERTIFICATE OF INCORPORATION

                                      OF

                                 MERCATA, INC.

     Tom Van Horn and Jon C. Engman hereby certify that:

     1.   The name of this corporation is Mercata, Inc. and the date of filing
the original Certificate of Incorporation of this corporation with the Secretary
of State of the State of Delaware is September 23, 1998.

     2.   They are duly elected and acting President and Secretary,
respectively, of Mercata, Inc., a Delaware Corporation.

     3.   The Certificate of Incorporation of this corporation is hereby amended
and restated to read as follows:

                                      I.

     The name of this corporation is Mercata, Inc.

                                      II.

     The address of the registered office of the corporation in the State of
Delaware is 30 Old Rudnick Lane, City of Dover, County of Kent, and the name of
the registered agent of the corporation in the State of Delaware at such address
is CorpAmerica, Inc.

                                     III.

     The purpose of this corporation is to engage in any lawful act or activity
for which a corporation may be organized under the General Corporation Law of
the State of Delaware.

                                      IV.

     A.  This corporation is authorized to issue only one class of stock, to be
designated Common Stock. The total number of shares of Common Stock presently
authorized is ten million (10,000,000), each having a par value of one-tenth of
one cent ($0.001).

                                      1.


                                      V.

     A.  The management of the business and the conduct of the affairs of the
corporation shall be vested in its Board of Directors.  The number of directors
which shall constitute the whole Board of Directors shall be fixed by the Board
of Directors in the manner provided in the Bylaws.

     B.  Subject to paragraph (h) of Section 43 of the Bylaws, the Bylaws may be
altered or amended or new Bylaws adopted by the stockholders entitled to vote.
The Board of Directors shall also have the power to adopt, amend or repeal
Bylaws.


                                      VI.

     A.  The liability of the directors for monetary damages shall be eliminated
to the fullest extent under applicable law.

     B.  Any repeal or modification of this Article VI shall be prospective and
shall not affect the rights under this Article VI in effect at the time of the
alleged occurrence of any act or omission to act giving rise to liability or
indemnification.

                                     VII.

     The corporation reserves the right to amend, alter, change or repeal any
provision contained in this Certificate of Incorporation, in the manner now or
hereafter prescribed by statute, and all rights conferred upon the stockholders
herein are granted subject to this reservation.

                                     VIII.

     The name and the mailing address of the Sole Incorporator is as follows:

         Matthew F. Roberts
         c/o Cooley Godward LLP
         Five Palo Alto Square
         3000 El Camino Real
         Palo Alto, CA 94306


                                     ****
     This Amended and Restated Certificate of Incorporation has been duly
approved by a majority of the Board of Directors of this Corporation.

                                      2.


     This Corporation has not received any payment for any stock and these
Amended and Restated Articles of Incorporation have been duly adopted in
accordance with Section 241 of the Delaware General Corporation Law and is
executed, acknowledged and filed in accordance with Section 103 of the Delaware
General Corporation Law.

     In Witness Whereof, Mercata, Inc. has caused this Amended and Restated
Certificate of Incorporation to be signed by the President and the Secretary in
Bellevue, Washington this 8th day of October  , 1997.


                                        Mercata, Inc.



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


ATTEST:



By:  /s/ Jon Engman
     --------------
     Secretary

                                      3.