When I purchased my boat, I made sure to get it inspected by a professional. Try Doug at Bay City Marine in Hamilton or Top Gun Marine in Bradford. Also find out where the owner got the boat serviced; if at all, they might be able to help. If the owner wants to sell it, have him tow to the shop and pay for the inspection. Best $100 you can spend.
You should do all this before trying to speculate what its worth. I also agree with Bushart, make sure you take it for a spin too.