Hmm, when I bought a dragster from there, I sent the guy 500 to travel on, and had the money waiting for him.. No problems he just wrote out a purchase agreement and we both signed it.
I am from Canada and have purchased two different cars from the US and have had zero problems. The 2nd time the customs didnt even look in the trailer! I would say just make sure you have a piece of paper signed by both parties and you should be good to go!