We always recommend drafting up a Bill of Sale that describes the Make/Model/VIN of the vehicle being sold, the purchase price, and the terms of the transaction (ie date(s), if any additional items come with the car, etc.). Both the buyer and seller should sign and date the Bill of Sale and each should keep a copy for their records.
Your state’s DMV website may have vehicle Bill of Sale templates. Additionally, we recommend a simple and free service such as https://eforms.com/bill-of-sale/.