This Addendum is intended for use in the unincorporated portions of Contra Costa County. A purchase and sale agreement is a contract between a buyer and seller for transacting a property in exchange for a specific price.Our purchase and sales templates help lay down the required financial terms and pricing, ensuring a smooth transaction between sellers and buyers. It outlines the buying and selling of a particular item or asset, ensuring clarity on the price, terms, and conditions for completing that specific deal.