A Washington residential purchase agreement is used when making an offer to purchase real estate. The agreement initiates the negotiation process.Upon the terms and conditions set forth herein, Buyer agrees to buy from Seller, and Seller agrees to sell to Buyer, the real property in Yakima, Yakima County,. Description of The Property. A Washington purchase agreement documents the sale of a piece of real property for a specified financial figure. Learn how to fill out the Washington state vacant land purchase and sale agreement in our step-by-step guide! Seller agrees to sell to Buyer and Buyer agrees to purchase from Seller the following described real estate in. Moneys necessary to complete the sale in accordance with this Agreement. Filling out the PSA. Please note that form 21 is for houses.