I've sold domains on both platforms but I've found over the past year or so that Afternic has really picked up steam. The trick is you need to have your domains entered into their premium promotion level. This means they need to have a fixed buy now price, and they need to be registered at one of the participating Afternic DLS registrars. My domains now show up on GODaddy, Enom, Tucows, Register.com, Name.com, Netsol, Namecheap, and a bunch of others, it's really cool and it works. Not for everyone as I know guys who won't set fixed prices but for me this works great. If your domains aren't with a participating registrar you should move them, it's worth it.
Clam Domainer