I recently upgraded to the new iPhone, and sold my iPhone 4 to Gazelle. I was hesitant after reading a lot of negative reviews, but figured I'd give it a try anyway, as I wouldn't be too upset if I got nothing for it.
I had a great experience with Gazelle. I answered a few questions about the condition of my phone on their website, and they quoted me at $110. For a phone I bought for $100 2 years ago (w/ 2 yr contract), this was surprising to me. This was in line with other websites, including the Apple store, AT&T, and Amazon, except on the other sites, you had to spend the money at their store. I'd rather spend it where I want.
A few days later I received a box in the mail with instructions on how to ship them my old iPhone. I dropped off my package with my phone at the post office the next day. Just in case I took some pictures before I shipped it. About 3 days after that I received an email that they received my phone and would examine it soon. The next day I got another email that said they had looked at my phone and would pay me the full $110 they quoted. The deposit was made to my PayPal account the next day. From the time I initially signed up online to the time the money was in my account was 10 days. This includes sending the package both ways.
I will note that although my phone was 2 years old, it was in great condition. I always kept it in an otterbox and used a screen protector, so it had no scratches or damage. I used a q-tip to clean dirt out of the ports/niches before sending. I cleared everything off of the phone, set it to factory defaults and loaded iOS7 before sending as well. I'm wondering if some of the bad reviews sent in phones in bad shape. I will note that I did not send the power cord or headphones, which I was worried would bring down the price but apparently it didn't.
Overall I had a good experience and would recommend.