This Addendum is intended for use in the City of Oakland. Please also review the separate.Purchase and Sale Agreement. A purchase and sale agreement is a contract between a buyer and seller for transacting a property in exchange for a specific price. The purchase and sale agreement is a contract in a real estate transaction. Learn about this agreement and what happens after signing. This form is intended for use with the California Association of REALTORS® forms RPA "Residential. In real estate, a purchase agreement is a binding contract between a buyer and seller that outlines the details of a home sale transaction.