Homework, homework, homework. Know what you're buying!!!
1. Be certain the vessel you're considering is suited for your intended needs. Talk to people that have similar or the same craft and get their input on the pros/cons.
2. Check out the classifieds to find out what the current market is fetching for the boat you're considering. Unless you have a distressed seller, if the boat is way under market value, chances are there is something wrong with it.
3. Do a little research on the engine that comes with the boat. You can find plenty of info on the internet. General wear of engine is usually a function of use and maintanence. If the engine has an hour meter installed, it may provide a basis for evaluating its condition and perhaps need for major repairs.
4. Ask for service records.
5. Depending on the cost of the vessel, hire a a marine surveyor to go through the boat and engine. Although I've owned boats all my life, I wouldn't buy a used vessel until I've had a valuation survey done. There are plenty of things that even an experienced boat owner may overlook. Sometimes, a boat will appear fine on the outside, but underneath it may have severe problems which are costly to fix (rot, delamination, hull defects, etc.)
6. Look for corrosion. If the metal parts on the boat (espcially any metal parts below the waterline) appear to be eating away, it may be a sign that the boat has electrolysis problems that have been ignored for some time.
7. Check for blistering in the hull, softspots (in the deck as well), be certain electrical systems work and that the wiring is proper and in good shape.
8. If the seller is willing, ask to give the boat a sea-trial.
9. Be prepared with questions when you speak with the seller.
10. Have someone with boat experience go along with you to ask questions you may not be aware to ask the seller.
11. Due your due diligence regarding the paperwork on the boat. Ask for a copy of the title, check for tax liens, etc.