If you are located in United States, I would recommend you to visit NADA Guides, an internet website for evaluating the value of boats and other types of vehicles such as automobiles, personal watercrafts and other vehicles. NADA stands for "National Automobile Dealers Association". NADA Guides is a vehicle-listing website that publishes specifications, research material and pricing information.
Together with marinebroker.net and NADA Boats you can get the perfect selling price of your boat, sailboat, outboard motors and even trailers. The NADA values are updated every month in book and online form.
I hope you find this information valuable and I wish you a happy selling/buying your boat.