I have sold every iPhone I've had (3) and soon to be selling the 4th for almost as much as I have paid for them. Ebay.
I have kept this one the longest. I'ts a iP4 32BG.
Gazelle wants to give me $135.00
Ebay shows they sell easily for $250.00
And that's for a version that is soon to be 3 generations old.
The iP3G I sold for 15 less than I paid and it was 18 months old.