To reduce the chance of losing boat and/or money, always get the full name, address, telephone number/s and email address of a prospective buyer, and check them out as best you can before agreeing to anything. For instance, find a reason to send an email that needs a reply, and yet another reason to telephone the prospect and check how the telephone is answered.
If the purchaser lives near you, carry out a 'drive by' to see if the car that they drove to come view your boat is the one in the driveway. And if they're buying your boat 'blind' that is, they've never been to view your boat alarm bells should be ringing loudly. Have you ever bought a boat blind? Would you ever? If you wouldn't, why should they? Exactly.