Submittal of this form is not mandatory, however it is strongly recommended if the Contractor is a Dealer for the Vehicle specified in the Mini-Bid. 5. Both the seller and buyer of a vehicle are required to fill out a Car Sale Agreement to ensure a clear, legal, and mutually agreed-upon transaction.Our free sample car sale agreement outlines the responsibilities of both parties, payment terms and condition of vehicle transfer. Here's everything you need to know about a car sale contract so you're prepared when it comes time to make a purchase.