All of my online buying of used CDs is through amazon marketplace sellers. I used to not have a lot of trouble getting just what was advertised, in the condition advertised, but all that has changed for some reason. These days, almost all of the used stuff I buy that is isn't actually in the condition it was listed in. Sometimes it's better! Often it isn't, though. I get "Like New" stuff delivered in "Very Good" condition or cut-outs (that weren't listed as such) or club copies. When this happens, I promptly let the seller know and request a partial refund, one that I will be happy with. They either:
1. Agree and give me the discount I ask for, or
2. Give me a full refund (sellers with 100% ratings seem to handle it this way) or
3. They give me a smart ass answer and I simply file an A to Z claim (as Karl suggested) and get all my money back.
In the case of 3 (almost never happens), they get poor feedback (1 star); for cases 1 and 2, they get 5 stars.
So to me, buying on amazon is ideal. There is no risk. If I am not happy, I don't have to pay, that's amazon's guarantee. And as I described above, things almost never reach that point, as the sellers are usually willing to settle ahead of time.