I pay pretty close attention to boat values, especially stern drive open bows. My highly unscientific conclusion is that if a vessel is listed on a website like Boat Trader, it is because it is way overpriced, and the seller is either oblivious to value or buried in it. Why would you pay to list a desirable boat if you want a fair market price?
Conversely, a nice boat can be worth double the NADA, because the NADA doesn't distinguish between low hour fresh water boats, and high hour salt water boats, which are mostly worthless. Ebay is a good place to learn values from.