There is always a market for previous models, was wondering what I would get for an iPhone 5 16GB Black when the 5S came out, sold it 3 weeks later for $465.00 so it just depends on the model, condition, GSM draws more money than CDMA from Sprint and VZ.
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You will also be able to sell an AT&T Unlocked GSM iPhone for more money than a Verizon iPhone each year. I think VZ with their new pricing still charges $40 per smartphone regardless if your under contract or not. I moved to the new family plan on AT&T ( I was already with them so it was easy) so now I pay $15 per iPhone and $100 for 10GB of data, I was paying $241.00 on the old plan.