Insist on taking it for a spin before you buy, or even seriously consider buying. Make sure the power, turning, and ride is what you had in mind. For example, I test drove a Howard 21' a few years back and it looked like a pretty good package, however it turned like the Queen Mary.
Have the engine and drive device thoroughly tested by a marine engine professional. Trailer prices have soared in the past few years, so don't settle for a rust bucket with bad brakes; it will cost thousands to bring up to snuff.
Finally, have a fiberglass pro/boat builder crawl underneath and check the bottom. They can tell what's important. A few years back I was considering buy a Spectra, and it was a beauty. The owner had rebuilt the motor and outdrive, repainted the boat and trailer, killer stereo, etc. I took it to a local boat shop and they pointed out a hull crack that would have cost thousands to repair. Now, the seller did not even know about it. The guys at the shop said his face turned white when they pointed it out to him. So it may not be a matter of someone trying to cheat you, but the pros know where to look for problems and what those problems cost to fix.