GameStop is giving a huge value over gazelle, even their cash value is better than gazelle and anyone else. You cannot compare the iPhone to the any android. The reason android phones are low is because there are a crap ton of models out there. More models = more choices = lower demand on each model.
Check gamestops website if you don't believe me. I traded in my gfs iPhone for her. She got $320 for her 4S 64gb without sending a phone in waiting for it to sell on the bay/scamlist