I've bought about 100 or so discs off Amazon used so far, and it's about a 95% success rate so far. Here are the problems I've run into:
.Bad condition description - had a couple of discs that had staining, rips, scratches, etc. when listed as Very Good (my usual minimum)
.Surprise warehouse editions! - Policy states that they must indicate if the CD is a BOTM Club edition or another kind of re-print. Sometimes the seller doesn't indicate this
.Wrong edition - This is Amazon's fault for how they lump together multiple titles. You want the remaster from 1990, but you get the original from 1980.
That's about all I can drum up from ~5 failures
Universally sellers have been quick to say "keep the disc free" when I complain, and I get me a free coaster! On another note, it seems that having a small remainder/hole-punch taken out of the UPC code area is acceptable as "New" even from Amazon proper, which I think it a bit sketchy. Oh well.
If you strongly desire a specific edition of a disc, I've found Discogs useful.