Well if you can see the boat in person and talk to the person on the phone number they gave then go see it. Ask to see the paper work and verify it against their drivers license. If it is all good, pay them in cash and part ways with a handshake, and drive off with your new boat in tow
To some people it is not about the money and they don't care. If it is a good deal, you need to jump on it the same day.