Find a friend with a boat!
I love boats. I grew up with them, and bought my own first boat when I was 18 years old. last year, I got a new Boston Whaler, and if there is one thing that I can tell you, THEY ARE FUCKING MONEY PITS!
Every single time that bitch goes in the water, I'd say it probably costs me about $300, between the gas, oil, washing the bitch afterwards, the beer for the trip, food, etc.
If you end up paying 19,500 for that boat, you'll probably have to hold onto it for 4 or 5 years at least to get your moneys worth out of it if you decide to sell it. I agree with an earlier poster that said, somewhere in the ballpark of 16,000 is a little more realistic.
i also agree to ask for some sort of verification and warranty on the motor rebuild. Every retard with a boat for sale says that the motor has been either rebuilt or replaced, when it reality, most of the time, they havn't.
Boats can be a lot of fun, but it's extremely easy to get fucked HARD on a private party sale, so make sure you try to get as much documentation as you possibly can. Even then it might not hurt to let the auction run its course and throw out a real low ball offer of somewhere around 12K just to see what he says.