Just write on paper, I sold (year make model), vin # , to (name of buyer) . No warranty as is condition. And price. Of course buyer will always want lower $ for sales tax purposes. DMV will compare price to NADA or Kelly Blur Book value. If it seems out of wack, they will fight him not you. Both sign . Depending on year of vehicle may need Affidavit of Sale forms from DMV.