I've done this every time I've purchased a new iPhone. (Not every year; I wait until I'm out of contract.)
Why? I have no need to keep an old phone around, Gazelle was one of the first places to offer a guaranteed price for a long period of time, Gazelle is very easy to use, and Gazelle is reliable.
There are places where you can get more money, but they take more time, are more hassle, or seem more risky. In my opinion, of course.