A lot of great advice on what to look for in a boat. Once you get comfortable with what you want, you can get a sense for whether the price is reasonable for a used boat by going to one of a couple websites.
www.boattrader.com allows you to search boats listed nationally by type of boat, size, manufacturer, model year, etc. While you would not likely travel these distances to buy your boat, it can give you an idea of what people across the country are asking for the boat you want to buy. Keep in mind, though, that the boats are not always comparable. Boats in the south often have more hours on them due to longer seasons. Some boats have spent their lives in salt water, etc. It just gives you a range though to see if what you are going to pay is in the range.
www.nadaguides.com also has used boat section where you can see what NADA says a used boat is worth (rather than the asking prices you find on boattrader.com).