Some good, standard online options have been mentioned.
But, for me personally, the novelty of the 'Net and some of the serendipity of online auctions have given way to tedium--the tedium of posting an auction, monitoring its progress, administering all the e-mail, going to the post office to ship and sometimes (albeit rarely) having to deal with, um, less-than-stellar members of the ebay community.
And, if these other online options that buy discs outright are like Wherehouse.com, I don't like that they retain whatever discs you mail to them which they don't buy even if you were quoted a price online before shipping.
This issue is one of the bigger reasons I'm lamenting
Wherehouse's Chapter 11 filing. They were/are the biggest major player in the preowned CD and DVD market.
-p