I would not use any of them. You will be better off selling your iPhone 4S by yourself. Advertise on Craig's list or your local Green Sheet for sale paper. Just be careful who you sell it to. Meet the prospective buyer in person at a public place and only accept cash. That's how I sold my MacBook.
I had a great experience on eBay. It was actually the first thing I have ever sold on that website, and I got $180 for my iPhone 3GS when the 4S was released. Darn near paid for my upgrade since I was able to re-up with AT&T! Of course they took a small fee, $20 or $25, but I was blown away at what someone was willing to pay for an iPhone that old! These devices really hold their value! I LOVE APPLE!
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