It may not be a requirement in the state where you register the vehicle, but it might be worth your time to type up a "bill of sale" for both parties to sign, date, etc. when you are face to face and close the deal. You can get a boilerplate copy online to "fill in the blanks" if you choose. My state (New Mexico) requires one. It was a pain to get later because I bought the vehicle used from a private party. At the time it was licensed in Washington State and it was parked in Arizona. Then, I moved to New Mexico to register the vehicle. It took two weeks to track the previous owner down, in Washington, and get the required paperwork. It wasn't a problem, but, it could have been easily solved if I knew at the proper time.
Chum lee