(One reason people might not do this with phones is that we think of them as being worth much less than they actually are. A new top of the line smartphone costs about $600, but is advertised as "$100, with contract". This is a sneaky way of saying "$100 down, we'll loan you the rest, and you'll pay it back two to three times over during the next two years.")

Update 2014-01-21: I sold my Galaxy SII a few weeks ago when I got a new phone. It was a little over two years old, and I got $150 for it via Craigslist. It helped that it was unlocked and GSM-compatible. One annoying thing was the buyer wanted to test it, but didn't have an appropriately sized SIM. We ended up putting in their SIM just right so it stayed in place while being the wrong size. In the future I'm going to be clear to buyers that the phone takes a SIM of size X and if they want to test it they need to bring an appropriately sized SIM.