KEH and internet sales giant B+H photovideo both buy and sell used equipment. Send them a description of your item and they will send you an estimated value. They test functionality and condition and either send you a check if your description is accurate or propose a lower price which you can accept or reject. Shipping both ways is free.
I have bought and sold stuff with KEH and have been happy with them. B+H photovideo has a stellar reputation and I'm sure that they are fair as well.
Advanced RV Sprinter