I've bought quite a few used CDs from Amazon sellers, as well. newbury-comics and MovieMars for sure, and will second JohnK's recommendation.
Newbury Comics is a Southern New England (So Neng, Ken?)
brick and mortar chain with about 15 stores. One is 3 miles from my house and I (too frequently) frequent another 5. It's great fun to flip through all that stuff in-the-flesh and be surprised from time to time.
From Amazon, I have never had any problems with the actual used cd's, though the jewel cases may have been abused.