Old/Used CDs

As a newbie to the list, what are the group's recommendations for a good 
source for used CDs? I've been disappointed with the selection at
Parnassus 
and Amazon. They never seem to have my wants. Search of newsgroups has 
produced relatively little. Is there anything like an e-bay for classical 
CD's where one could match buyers and sellers? For me the pleasure is NOT
in 
the search....