You, the Buyer (and Co-Buyer, if any), may buy the vehicle below for cash or on credit. Documentation. Your Motor Vehicle Purchase Agreement (the "Agreement") is made up of the following documents: 1.In this post, we'll explore everything you need to know about car purchase contracts, cancellation agreements, and your rights as a buyer. Shop Diamond Chevrolet of San Bernardino Chevrolet Trax Express. In California, there is no authoritative return policy on used cars. Once a purchase contract is signed, it is difficult to reverse it.