Find out what documents you will need to provide proof of sale and ownership if you are buying, selling, or gifting a vehicle. The car exchange agreement is structured similarly to a typical purchase and sale agreement.Both exchangers should complete the one form, one member on the left and the other member on the right. This document is a car exchange agreement between a car owner and user. This form certifies the purchase price and determines the sales tax you must pay when registering the vehicle. Information you can expect to fill out includes the car's VIN, make, and model, the name and permanent address of each party involved, and the sales amount.