A friend of mine recently bought a F150 with similar date. The dealer sold it to him as a certified used vehicle, at that pricing. My advice is to put all the options into NADA guide as a certified used vehicle, not a new, and see how it comes out. Also get a price on Edmunds.com. It may not depreciate as much as you think since it's a F250.
A lot of times a vehicle like that hasn't sold because it's an odd color for the area, or is missing some option a lot of people want. Check the sticker and package details carefully. My friends had a tow package but did not have the brake controller, which is pretty lame.