Find out what documents you will need to provide proof of sale and ownership if you are buying, selling, or gifting a vehicle. 1 If you're financing your purchase ; 3 The actual vehicle purchase agreement ; 4 Buyer's trade in certification ; 5 Warranty disclaimer and documentary fee notice.1. Enter the buyer's name and contact information. This form certifies the purchase price and determines the sales tax you must pay when registering the vehicle. Information you can expect to fill out includes the car's VIN, make, and model, the name and permanent address of each party involved, and the sales amount. This document outlines the terms of the sale between the buyer and seller, including the price of the car, any additional fees, and the date of delivery. Step-by-Step Vehicle Purchase Agreement Breakdown. County of Harris. §. This is to certify that on the.