My advice, don't believe a thing anyone tells you over the phone or in an ad.
Make them show you the paperwork first and make them show you their I.D. If any of its missing or their I.D. doesn't match the paperwork, walk!
Regardless of how righteous or nice the seller seems in your correspondence, never carry cash to your meet. Do the transaction with the seller at the teller's window at a bank. There will be video of the transaction. If they refuse to go in to do the transaction, walk!
If possible ( and I've used this one to chase off a few characters when selling my boats. ) ask for the viewing to be at a local police department parking lot and don't be shy about walking into the p.d. and check-in with them, letting them know why you're in their lot. Most will give you kudo's for using your head.
If you don't know much about the type of boat you're after, take someone with you who does.
Here's a simple one, what size ball do you need and what type of electrical connection does the trailer take? Go prepared to trailer it home.
Honestly, I've never bought a used boat. To me, it's simply easier to hold off on a purchase, put together more funds, think it over a bit longer, learn more, research more and get what I really want. Also, I have never had a new boat shipped to me, I've always ordered it, waited for the build and drove ( as far as three states away ) on an extended weekend to pick up my new boats. It's kind of cool to walk the floor of the boat works, get to meet the crew who built your boat, the company owner, etc. and to get that " You're one of the family " feeling.
I realize that's not for everyone, but for first time buyers I think it's a safer way to go. Let the people who design and build them walk you through your first boat.
...and as the seller of used boats, I'm a lot more comfortable selling a boat to someone who knows what their buying and has experience rowing.
Whichever way you go, good luck.