Sorry to hear about your bad experience. Unfortunately, there seem to be more and more of these "basement slabbers", which are often the actual sellers of the coins. They are often horribly overgraded, and sometimes even cleaned, scratched, etc. and passed off as problem-free coins.
The best advice I can offer you is to stick with reputable grading services exclusively. Those would be PCGS, NGC, ANACS and ICG. Any other slabbed coins that I consider buying are treated as raw coins. I have bought some coins that were in (what are in my opinion) semi-junk holders like PCI and SEGS that I thought were accurately graded and a good deal.
But above all, buy the coin and not the slab. Even the big four that I mentioned can be wrong, although they are far more accurate, consistent and respected in the marketplace than the scores of cut-rate and "wanna-be" slabbers. I wish there was something more that could be done about the outbreak of bad grading in slabs on auction sites. ~Jim