I have used secondspin and abundatrade. Read through the "how to sell" info on both, enter in the CDs on both, and figure out which one will yield the most for you.
I've been SHOCKED at how much we've gotten for some CDs. The Conan the Barbarian soundtrack was nearly $20! The "3 Men and a Little Lady" was more than Conan. I don't know who is out there buying those things for that much (and more, since the two sites resell them), but depending on who is out there wanting things, you could be really surprised.
Paula Abdul CDs, however, are worthless. Poor Paula.
I have no interest waiting for the ultimate buyer to come along and buy my stuff. I'm happy to let the middleman do the hard work of finding that mythical person. I'm good with those two sites, and seriously have to get my rear into gear on selling the rest!
The other thing is...read the info that's on at least one of the sites I mentioned on CD "rot". And check each of your CDs for it BEFORE sending it in. They won't buy them if there's rot, and they'll either dispose of the CDs for you or you get to pay to have the CDs sent back. So check out each CD carefully. Also know that broken cases will have a fee associated with them, so you might want to replace broken cases if you can get them for cheaper than what the sites will charge you. Doing all the reading first, and making sure your CDs and cases are in good condition before sending them in, is worth it.