I've sold 2 iPhones and 2 iPads on Swappa with absolutely no problem. Trade-in's to Apple are a bit of a rip. AFAIK, you can always get more from selling verses trading in. That said, about 2-3 months ago I had an IP6S that was having some problems. My policy is to never sell any electronic item that has problems without full disclosure. At that time, the 6SE's were selling for $100-$150 on eBay. Instead, I took the phone to an Apple Store to trade in for the latest SE.
My salesmen looked the phone over pretty closely (I think he might have even taken it to the backroom) and then he offered me $70 on a trade-in which I took in a NYM.