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.
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.