Waste of time trying to do a trade-in with the dealer. BTDT. Sell on CL or to a family member and you will be ahead. Better off to squeeze the dealer for lowest price on just buying a new unit. Maybe wait until fall when they want to clear out 2015 models?
When we ended up moving to a bigger & better TT, we ended up selling our first TT, a 20 footer, to my BIL. He was looking anyway and got a smokin' deal on ours plus I had done a lot of great mods (cost a few thousand $$). Dealer went by the blue book price based solely on model year and offered us peanuts - felt very insulted. Tried CL but bad time of year (Feb.-Mar.) plus too much competition. If you want to sell privately, you need to be prepared to wait. Time of year is important.