Wash. Rev. Code § 36.34.070
The board may advertise and sell used highway or other equipment belonging to the county or to any taxing division thereof subject to its jurisdiction in the manner prescribed for the sale of county property, or it may trade it in on the purchase of new equipment. If the board elects to trade in the used equipment it shall include in its call for bids on the new equipment a notice that the county has for sale or trade-in used equipment of a specified type and description which will be sold or traded in on the same day and hour that the bids on the new equipment are opened. Any bidder on the new equipment may include in his or her offer to sell, an offer to accept the used equipment as a part payment of the new equipment purchase price, setting forth the amount of such allowance.
In determining the lowest and best bid on the new equipment the board shall consider the net cost to the county of such new equipment after trade-in allowances have been deducted. The board may accept the new equipment bid of any bidder without trading in the used equipment but may not require any such bidder to purchase the used equipment without awarding the bidder the new equipment contract. Nothing in this section shall bar anyone from making an offer for the purchase of the used equipment independent of a bid on the new equipment and the board shall consider such offers in relation to the trade-in allowances offered to determine the net best sale and purchase combination for the county.
RCW 36.34.070