A bill of sale is more akin to a receipt rather than a contract. Since it is such a simple document, there is no way to truly enforce it. If the bill of sale is the only document you use in a transaction, it is best to ensure there is enough detail on it to make the transaction enforceable.
A bill of sale isn't the same as a sales contract. A sales contract is drawn up and signed before a transaction. It lays out the terms of the transaction before buyer and seller come to a final agreement.
It should be signed by both seller and buyer. Each should get a copy. While it is not required by law, it's a good idea to have a Bill of Sale signed by both seller and buyer. That documents the fact that the vehicle was in fact sold.
Getting a Copy of the Contract Under TILA, the dealer is required to give the customer a copy of the contract to keep at the time the customer signs the retail installment sale contract whether you want to incur the debt on these terms.