When you consider the fact they have to resale the unit and make a profit they're not a resale. Its like trading in a car to a dealership. You'll get far less money from them, then if you went and sold the car yourself. The convenience factor is important and so saving time and headaches count for something.
Whether you decide to use them or not is your decision and since you think they're a rip off then its safe to find a different solution.
I hear of too many issues with selling phones online now a days, to the point I'd rather not deal with those hassles and headaches.