A Pennsylvania vehicle bill of sale is a legal document formalizing the sale and purchase of a motor vehicle between a buyer and seller. I'm new to car financing.What got me confused is that my bank wants purchase agreement or bill of sale before sending out a check to the dealer. The buyer needs to fill out Section D on the front of the title, including lien information if they are financing the vehicle. How is the sales tax charged on the purchase of a car with a trade-in?