Listen to these guys, they are savvy on buying a boat; a good used boat is the way to go, if you follow the rules.
Getting a reputable mechanic, one who specializes in the engine brand you are buying, can be the safety net between a good deal and a sucker deal. He can pull the plugs and inspect the cylinder walls with a fiber-optic viewer, check compression, test the ignition system, and check for known inherent problems with the particular engine that you are getting.
Buying in the late fall and winter will usually get you a better price and the mechanic’s business will be slower, giving him the time to do his inspection in more detail.
Again, be sure of the mechanic that you use; he is your only protection that you have from you taking home someone else’s problem. Don’t be in a hurry; the hassle will save you a lot of grief later on.
Good luck.
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Never look down on someone unless you're helping them up.