<PAGE>

                                                                    EXHIBIT 10.6

THIS SUBLEASE, dated December 3, 1999, between GMAC MORTGAGE CORPORATION, a
Pennsylvania corporation, with its principal address at 100 Witmer Road,
Horsham, Pennsylvania 19044, hereinafter referred to as Sublessor, and MERCATA,
INC., a Delaware corporation, with its principal address at 110 - 110th Avenue
N.E., Suite 300, Bellevue, Washington 98004, hereinafter referred to as
Sublessee,

                             W I T N E S S E T H:

Sublessor hereby lets to the Sublessee and Sublessee hires from the Sublessor
the following described premises:

     Approximately 2,912 rentable square feet known as Suite 100 located at
     110 - 110th Avenue, Bellevue, Washington,

to be used only for office purposes, for a term commencing December 1, 1999 (or
upon the date of first occupancy, whichever occurs later), and expiring May 31,
2000, at the yearly rent of EIGHTY-ONE THOUSAND FIVE HUNDRED THIRTY-SIX AND
04/100 DOLLARS ($81,536.04), payable in equal installments of $6,794.67 in
advance on the first business day of each and every month during the term,
together with monthly payments of $800.00 for furniture rental.

THE PARTIES HERETO COVENANT AND AGREE WITH EACH OTHER AS FOLLOWS:

FIRST - PAYMENT OF RENT: Sublessee shall pay the rent at the times and in the
manner aforesaid to General Motors Corporation, c/o Corporate Accounting, 4
Walnut Grove, Horsham, PA 19044, Attention: Leslie Kornfeld.

SECOND - REPAIRS: Sublessee at Sublessee's expense shall keep the premises in
good repair, ordinary wear and tear, repairs to the roof, exterior of the
building and structural repairs excepted, unless such repairs are made necessary
by the act or negligence of the Sublessee and at the expiration of the term to
remove its goods and effects and peaceably yield up the premises to the
Sublessor in as good condition as when delivered to Sublessee, ordinary wear and
tear, damage by fire, the elements, act of the public enemy or casualty
excepted; all notices to quit or vacate being hereby expressly waived, any law,
usage or custom to the contrary notwithstanding.

THIRD - COMPLIANCE WITH LAWS, ORDINANCES, AND ENVIRONMENTAL: Sublessee shall
promptly comply with all laws, ordinances, requirements, and regulations of the
federal, state, county, municipal, and other authorities, the fire insurance
underwriters, and any insurance organizations or associations; except that
Sublessee shall not be required to make any alterations to the exterior of the
building, or alterations of a structural nature. Sublessee represents and
warrants that it will comply with all specifically applicable Environmental Laws
in connection with its use of or operations at the property, and that Sublessee
warrants and agrees that it will indemnify and hold Sublessor harmless from any
claim to which Sublessor may be subjected to the extent such claim results from
Sublessee's breach of any of its representations or warranties.

FOURTH - VIEWING PREMISES: Sublessee shall use the premises exclusively for the
purpose set forth herein and during the last three (3) months of this Sublease,
or any extension thereof, and permit the Sublessor to display the usual "To Let"
signs and to show the premises to prospective tenants. Sublessee further agrees
that at any time during the term Sublessor, Sublessor's landlord, or their
agents, may enter the premises for the purpose of examining the condition
thereof, or to make repairs in any part of the building, but in making such
reservation, Sublessor does not assume any liability for the care or supervision
of the premises or appurtenances.

FIFTH - ALTERATIONS AND ASSIGNMENT: Sublessee will not make or permit to be made
any alterations or additions to said premises, nor assign, mortgage, or pledge
this Sublease, nor sublet the whole or any part of the premises without
Sublessor's written consent. Consent by Sublessor shall apply solely to the
particular transaction consented to and shall not constitute a waiver by
Sublessor of the provisions of this Sublease.

SIXTH - INSURANCE: Sublessee will not leave the premises unoccupied during the
term, nor by any act of commission or omission cause an increase in the rate of
insurance or the cancellation of any insurance policy. In the event of any
increase in the rate of insurance caused by Sublessee's occupancy, Sublessee
agrees to pay on demand the amount of any such increase, and in default of such
payment, such amount may be added to the next installment of rent as additional
rent. Sublessee shall, at its sole cost and expense, obtain and maintain during
the term of the Sublease, Comprehensive General Liability Insurance, including
Blanket Contractual Liability coverage, with limits of not less than Two Million
Dollars ($2,000,000) Combined Single Limit for Personal Injury and Property
Damage; Comprehensive Automobile Liability Insurance covering all owned, non-
owned and hired vehicles with limits of not less than One Million Dollars
($1,000,000) Combined Single Limit for Personal Injury and Property Damage; and
Statutory Workers Compensation and Employers Liability coverage with limits of
not less than $250,000. Sublessee shall deliver to Sublessor upon request a
certificate evidencing such coverages. Such insurance policies shall provide for
no cancellation or material alteration without thirty (30) days prior written
notice to Sublessor.

SEVENTH - SIGNS AND SIDEWALKS: Sublessee shall not install any awnings,
advertisements, or signs on any part of the premises without Sublessor's written
consent and will keep the sidewalks free from ice, snow, and all obstructions.

EIGHTH - UTILITIES: Sublessee will make its own arrangements for the supply and
payment of all services including without limitation gas, electricity, and
water.

NINTH - INDEMNIFICATION: Sublessor shall not be responsible for any defect or
change of condition in said premises, nor for any damage thereto, not to any
person, nor to goods or things contained therein due to any cause whatsoever
except the act or negligence of the Sublessor, and Sublessee will indemnify
Sublessor from any claims, demands, and actions arising in connection with
Sublessee's use of the property, or the use by any person occupying said
premises during the term hereof, or by reason of any breach or non-performance
of any covenant herein, or the violation of any law or regulation by Sublessee.

                                      1.
<PAGE>

TENTH - FIRE AND CASUALTY: If the premises shall be so damaged by fire, other
casualty, or act of the public enemy so as to be substantially destroyed, then
this Sublease shall terminate and any unearned rent paid in advance by Sublessee
shall be apportioned and refunded to it, but in case the premises are not
substantially destroyed, Sublessor will endeavor to have its Lessor restore the
premises and a just proportion of the rent shall abate according to the extent
to which premises have been rendered untenantable until the premises have been
restored. The Sublessee agrees to give the Sublessor immediate notice of any
damage to the premises.

ELEVENTH - DEFAULT: In the event Sublessee fails to perform or observe any of
the covenants contained herein on its part to be observed and performed for ten
(10) days after notice by Sublessor, (a) Sublessor may forthwith terminate or
cancel this Sublease by notifying Sublessee as hereinafter provided, and upon
such termination or cancellation, Sublessee shall be liable to Sublessor for all
damages Sublessor sustains by reason of Sublessee's breach of covenant and of
such termination or cancellation; or (b) Sublessor may forthwith re-enter the
premises without notice and upon re-entry may let the premises or any part
thereof as agent for Sublessee and receive the rent therefor, applying the same
first to the payment of such expense as Sublessor may be put to in entering and
letting the premises and then to the payment of the rent and the fulfillment of
Sublessee's covenants hereunder, and Sublessee agrees to pay and shall be liable
for amounts equal to the several installments of rent as they would, under the
terms of this Sublease, become due if no default had occurred, whether the
demised premises be re-let or remain vacant in whole or in part for a period
less than the remainder of the term, or for the whole thereof, but Sublessee
shall be entitled to be credited at the end of each month with any net amounts
actually received by Sublessor during such months for the use or occupancy of
the demised premises or any part thereof; provided, however, that all sums paid
and liabilities incurred by Sublessor for any of the purposes aforesaid (which
Sublessee also agrees to pay and shall be liable for) shall have been first paid
in full to Sublessor, either directly by Sublessee or out of moneys actually
received for renting said demised premises after Sublessor shall have received
undisputed possession thereof, and the maintenance of any action or proceeding
to recover possession of the premises or any installment or installments of rent
or any other moneys that may be due or become due from Sublessee to Sublessor
shall not preclude Sublessor from thereafter instituting and maintaining
subsequent actions or proceedings for the recovery of possession of the premises
or of any subsequent payment or payments of rent or any other moneys that may be
due or become due from Sublessee or Sublessor. A waiver by Sublessor of any
breach or breaches by Sublessee of any one or more of the covenants or
conditions hereof shall not bar forfeiture or any other rights or remedies of
Sublessor for any subsequent breach of any such or other covenants and
conditions.

TWELFTH - ADDITIONAL RENT: If Sublessor shall make any expenditure for which
Sublessee is responsible, or if Sublessee shall fail to make any payment which
Sublessee is obliged to make hereunder, then the amount thereof may, at
Sublessor's option, be added to any installment of rent then due or thereafter
becoming due.

THIRTEENTH - CONDEMNATION: In the event the premises or any part thereof are
taken or condemned for a temporary or permanent public or quasi-public use,
Sublessor may, at its option, terminate this Sublease and in such event any
unearned rent paid in advance shall be returned to Sublessee.

FOURTEENTH - DEMOLITION: If any authority having jurisdiction shall decide that
the building or buildings should be demolished and removed, then forthwith upon
such decision being made, the Sublessee shall vacate the premises and this
Sublease shall cease and come to an end and any unearned rent paid in advance by
Sublessee shall be apportioned and refunded to it.

FIFTEENTH - NOTICES: That all notices to be given hereunder by either party
shall be in writing and given by personal delivery to the Sublessee or to one of
the executive officers of the Sublessor or shall be sent by certified mail
addressed to the party intended to be notified at the post office address of
such party last known to the party giving such notice and notice given as
aforesaid shall be a sufficient service thereof. Provided, however, that it is
mutually agreed that the Sublessor appoints the General Director and all
Directors of Worldwide Real Estate, General Motors Corporation, Mail Code 482-
309-939, 485 West Milwaukee Avenue, Detroit, Michigan 48202, as its agents, and
that any of them may give or receive all notices to be given hereunder, and
notices shall be sent to any of them and not otherwise. The right is hereby
reserved by the Sublessor to countermand such appointments and make others
consistent herewith, due notice of which shall be given by the Sublessor to the
Sublessee.

SIXTEENTH - TERMINATION: If at any time proceedings in bankruptcy, or pursuant
to any other act for the relief of debtors, shall be instituted by or against
Sublessee, or if Sublessee shall compound Sublessee's debts or assign over
Sublessee's estate or effects for payment thereof, or if any execution shall
issue against Sublessee or any of Sublessee's effects whatsoever, or if a
receiver or trustee shall be appointed of Sublessee's property, or if this
Sublease shall by operation of law, devolve upon or pass to any person or
persons other than Sublessee personally, then and in each of said cases,
Sublessor may terminate this Sublease forthwith by notifying Sublessee as herein
provided. Upon such termination all sums due and payable or to become due and
payable by Sublessee shall at once become due and payable.

SEVENTEENTH - SUBLEASE: This is a Sublease and the Sublessor's interest in the
premises is as Lessee under an underlying Lease made by 110 Atrium Place
Associates, LLC, dated October 19, 1998, a copy of which, initialed for
identification, is attached hereto. This Sublease is expressly made subject to
all the terms and conditions of said underlying Lease and the Sublessee agrees
to use the premises in accordance with the terms of said underlying Lease and
not do or omit to do anything which will breach any of the terms thereof. If
said underlying Lease is terminated, this Sublease shall terminate
simultaneously and any unearned rent paid in advance shall be refunded to the
Sublessee.

EIGHTEENTH - QUIET POSSESSION: Sublessor hereby covenants that Sublessee, upon
paying the rent as herein reserved and performing all the covenants and
agreements herein contained on the part of the Sublessee, may quietly enjoy the
premises, except as herein otherwise provided, and subject, however, to the
terms of the Sublease to Sublessor, and to the terms of any mortgages which may
now or hereafter affect the premises.

NINETEENTH - WAIVER OF SUBROGATION: Sublessor and Sublessee waive all rights,
each against the other, for damages caused by fire or other perils covered by
insurance where such damages are sustained in connection with the occupancy of
the leased premises.

The covenants and agreements contained in the foregoing Sublease are binding
upon the parties hereto and their respective heirs, executors, administrators,
successors, legal representatives and assigns.

                                      2.
<PAGE>

IN WITNESS WHEREOF, the Sublessor has signed and sealed this instrument this
9th day of December, 1999, and the Sublessee has signed and sealed this
---        --------
instrument this _____ day of __________, 1999.

In the presence of:                     GMAC Mortgage Corporation

/s/ Kathleen M. Manga                   By: /s/ L.F. Whatley
------------------------------              -----------------------------

   Kathleen M. Manga                    Attest: /s/ Cheryl Pence Lindeman
                                                -------------------------

In the presence of:                     Mercata, Inc.

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

                                        Attest: /s/ Jon Engman
______________________________                  -------------------------
                                                Secretary

                                 C O N S E N T

110 ATRIUM PLACE ASSOCIATES, LLC, the Lessor under the underlying Lease dated
October 19, 1998, and referred to in the within Sublease dated December 3, 1999,
hereby consents to the making and the terms of the within Sublease with the
express understanding that such consent shall not operate to release GMAC
Mortgage Corporation from any of the Lessee's covenants in the underlying Lease.

Dated:_______________________

                                    110 ATRIUM PLACE ASSOCIATES, LLC

                                    By __________________________________

                                      3.