Well, since the iPhone 5S and 5C were announced, there will be a plethora of older iOS devices available for sale. I have a 5, and my wife has a 4S, and I'd like to use this opportunity to upgrade to the newer devices, primarily to get supported LTE band support. It also never hurts to have the latest/greatest new devices :-)
So what to do with the old devices ? I'm going to give Gazelle a try. Essentially, you tell them what device you have, and they then ship a box to you which you put your device into, and at the other end out pops a cash offer to you. This seems to be the most effective and easy way to trade in your old devices.
I've submitted my requests for two devices, so lets see how the service works.