New York Times:
You can sell old iPhones — a lot of Android phones, too — to a number of online companies. Gazelle.com, cashforiphones.com, usell.com and icracked.com are just a few of them. Amazon and eBay are also vibrant marketplaces for used iPhones. I don’t know which one offers the best price or which are the most reputable. I don’t have enough used iPhones to do comparison testing.
But looking at the prices offered for the used iPhone 5s may offer some insight into what phone to buy next if you intend to sell it after a few years of use.
I’ve used Gazelle for several iPhone sales. Not the best price but definitely the easiest and less hassle.