Nie jest to umowa jako taka, więc nie można tłumaczyć "mowa kupna-sprzedaży". Jest to potwierdzenie zawarcia transakcji, więc chyba można tak przetłumaczyć.

Instructions
This is a New York State sales tax return. The new owner's social security number, taxpayer identification number (TIN), or federal employer identification number (EIN) is required.
Use this form to report the purchase of a vehicle or boat when sales tax was not collected at the time of purchase or when the vehicle or boat was received as a gift. If the donor/seller is not required to complete Section 6, the new owner must have a copy of the bill of sale signed by the seller.
http://www.nydmv.state.ny.us/forms/dtf802.pdf