To be a valid contract, both seller and buyer must sign this contract in due form with witnesses and execute it on a valid stamp document, in accordance with the state of performance of this agreement. The vehicle sales contract helps to avoid disputes regarding the sale of the vehicle in the future. Important details regarding the vehicle may be included in this Agreement. Some states require the addition of a sales and use tax to the purchase price of the personal property sold. Be sure to indicate in your purchase and sale contract who is responsible for these taxes. A sales contract is signed before the exchange of goods or money. It is an agreement between the parties to enter into a future transaction and documents the details of what that transaction will be.