When selling a vehicle, the seller must sign and handprint their name on the proof of ownership document. A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments.Declare my cash deposit and trade-in, as referenced on page one forfeited; B. " The sale of a motor vehicle for the buyer's use or another's use from which the buyer derives a benefit or satisfaction. Online purchase from dealer: send down payment before paperwork signed? Currently in the process of buying a new car online.