I think the bottom line is you should do your own homework when buying a boat. I wouldnt rely on a website to tell me how much a boat is worth.
Shop around, ask questions, look at many sites and compair, get a feel for the market.
....and if someone has an asking price youre unhappy with, find another boat.
deviatedclothing: In another thread you mentioned you were looking for a Daytona. These are very saught after hulls and have a demand, this is why they might be priced a bit higher then you expected and are priced much higher then NADA lists them.
Personally, I dont think you can compair KBB to NADA along the lines of jetboats. NADA does not have a solid database of boats. There was a large amount of boat hull manufactures in the 70's. It seemed as if every street corner in California had a shop pumping out hulls. Most were copies and rip offs of other well established hulls.
Today, there are only a handfull of companys still offering jetboat hulls. Thus why the used market has a large demand.
Good luck with your boat shopping!
-Goad