I too have had marginal experience with Thriftbooks. I find that I can really only purchase books from them when listed as "Like New" and that is even a risk. It doesn't help that the books are always thrown in a thin plastic bag with no protection whatsoever. I bought a "Very Good" hardcover last year that arrived with the cover RIPPED. Yes the hardcover board itself was RIPPED about 2". There was no damage to the shitty little bag it was in either.

I have had some OK purchases with them but I find the condition is always much worse than listed. I just assumed this is how they are. It's interesting to see others have had seemingly opposite experiences.