This form certifies the purchase price and determines the sales tax you must pay when registering the vehicle. Find out what documents you will need to provide proof of sale and ownership if you are buying, selling, or gifting a vehicle.This legal document includes details about the vehicle, including its make, model, year, VIN, payment terms, and purchase price. Welcome to the Suffolk County Clerk's Office. At one time or another either directly or indirectly, every resident of Suffolk County deals with our office. Many New York counties or municipalities levy their own sales tax. The property and improvements which the Seller is agreeing to sell and which the Purchaser is agreeing to purchase is known as.