This is where I started and ended up with a great boat!http://www.bassboatcentral.com/buyingusedboat.htm
Print these out and study them. Take a compression gage, hose, plug socket and ratchet, muffs, and a gallon jug cut out at the top.
These items are very important. Never give the asking price. Let them stew on your offer for a week. Nobody is ever "solid" on a price. If they are, they will break when they see the boat will not sell for it and they will come around.
I have gotten people down from $200.00 - $4000.00 below asking price.
If they are solid on it, then keep looking. Don't have a hair trigger on the buy and look at the boat from stem to stern and look good. If you think you looked it over good, move on to the pumps, batteries, motor, then come back and look it over again really good until you are totally pleased with the boat. If you see something on the first once over, you will see 10 more things the second time around.
Point out everything you see to the owner and ask about where every scratch came from. They know, believe me. if they say they don't remember, you can't take anything they say to heart and move on.
Look at as many boats as you can. Let them know there are plenty of better boats out there cheaper than they are asking. Don't low-ball them and tell them the boat is junk. It is there property and they are proud of it, so there is no reason to be cruel. Treat them as you would want to be treated and offer a fair market price minus all the work that needs to be done for the boat to be where you would want it.
This is a huge investment, take it very seriously.
Dave