as with any boat, check the basics.
cracks, fiberglass or alum tanks, delamination, stringer condition, floor condition, gelcoat condition, does it run, check the wiring, check the upholstery. boats arent "what am i buying?" they're "what do i have to fix?" they can turn into HUGE money pits easily. ask about last oil change, last service, etc. if they took care of it they'll be able to just talk easily about the boat and that means they took good care of it. if things simple things like a $100 steering cable need replacing, chances are a LOT of things need some attention if you look hard enough. lastly check the trailer, are the bunks in good shape, fenders, tires, wiring, bearings/grease.