I've never had any problems selling any of my iPhone's, and I've done it right at the release time of the new devices every year like clock work...I haven't had a 16GB model since the 3GS, so most of my experience has been with selling the 32GB variants, and I always make exactly what I expect to every single year. The key is condition, but also offering things that people are really looking for...
- With the iPhone 3GS and iPhone 4, people were still kind of new to the jailbreaking world, so jailbroken models were cream of the crop...selling both as "already jailbroken" made for very easy sales.
- The iPhone 4S was more of the carrier pull and color...the AT&T phones are always the quickest sellers, but the white AT&T models with the 4S sold like hot cakes. I happened to have a black model, but it was unlocked (since I bought it at full retail like I've done ever since).
- The iPhone 5 was almost completely centered on being "carrier unlocked", and while you could do that using the online services, my iPhone 5 was, again, full retail so it was unlocked officially, and was a simple sell at that point.
...like we were discussing in the other thread, there really isn't much difference at all between the 16GB and 32GB models...and really, only a slightly less valuation on the 64GB at the 1 year point. Where you really start to see depreciation is at the 2-3 year mark...the 16GB and 32GB models hold at about the same level of value drop, but the 64GB model REALLY takes a hit at that point, dropping to just a hair over the 32GB model's valuation, and I think it's really because the 64GB (up to this point) have not been all that popular...which really made me laugh when so many people were clamoring for Apple to release a 128GB iPhone...as it was really a TINY little niche market that wanted it.
Just to put this out there, I've never had any of my iPhone's on the "market" for sale for more than about a day before I had a solid buyer lined up. I even found a buyer this year willing to wait until I had my new 5S IN HAND before he took possession of it...that is a rarity really, but it was someone I knew too so that helped.