Saw someone else mentioned Spun.com, which I've bought from many times. In terms of music selection I guess it's standard, nothing special there. But what I personally like about the site so much more than a ton of others (including Amazon.com!) is:
1) You can sell CDs to them. Their trade-in program rocks. If you only sell, of course you need to provide your own box and pay shipping, but if you buy 3 or more items in an order, they'll send you the box, and a pre-paid label! And did I mention you can sell your unwanted CDs? They give a fair credit, based on their inventory of the item...the higher the demand is, the more they'll give you credit for (timing becomes a science).
2) Very professional. They never use those USPS media mail packages that CD cases are liable to break in - always a mint corrugated box with bubble wrap to protect the contents, very well sealed, they do have a guarantee and as a prolific buyer, I can personally attest to it. CDs are ALWAYS in excellent condition, you wouldn't know they're used.
Shipping isn't free (but it is FAST - with their UPS Ground it's usually 3 or 4 days from their warehouse in Chicago to Denver/Boulder, where I live) and they never have special offers, which kinda sucks, but their trade-in program and professionalism are always a pleasure to deal with. Sorry this post was long, but I really like Spun.com...