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.
An installment contract is a single contract that is completed by a series of performances–such as payments, performances of a service, or delivery of goods–rather than being performed all at one time. Installment contracts can provide that installments are to be performed by either one or both parties.
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.