Many have asked about Gazelle and whether they're a good option for selling your used tech. The answer is absolutely yes. I've sold almost all my iPhones and one of my Android tablets on Gazelle and they've been stellar every time.
They ship you a free box with free return shipping, you put the phone in and drop it in the mail. You can opt for either cash or an Amazon gift card. The card gets you 5% more cash value. I always get the Amazon card because, ah, more money is more money than less money.
They also sell used, refurbished tech and it's good stuff with certifications and 30-day returns. I like them so much I became an affiliate and this is my affiliate link attached. That is all.