if you're considering it already, i'd say go ahead and sell it. The 5DIII is still young in it's life and the 7D is very old, but yet to be replaced. You definitely wont be disappointed in the 5DIII, it's a great camera. I believe the 7D should be replaced very soon(60D as well), and when that happens, the used price will drop.
Personally, i'd sell the 60D and keep the 7D, but hey, it's up to you.