How To Legally Buy Or Sell A Used Car It is important to complete an auto bill of sale form when selling or buyinga used car. For the seller, an auto bill of sale form provides proof thattitle to the used car has been legally transferred. For the buyer, the autobill of sale form documents the purchase of the used car. The auto bill ... Similar Editorial : Legally Eliminate Debt by Raylam. | Source : Used Car Auctions