General rule of thumb ( I believe ) will be about 10 to 20%. Consider that most used boats found in a dealership are trade in's then maybe that figure could fluctuate somewhat.
On the used boat looking, eBay would be a good starting point. There you'll be able to tell the supply and demand factor and the money people are willing to pay. Newspaper ads, boating magazines, Boat Trader, etc. are all great sources as well. Good luck !!
