I'll take a shot at this. I placed an order with MCS's eBay store a while back. They offered free return shipping, so I figured why not; if I was unhappy with the grading, I could return the books. I would sat at least half of the books were overgraded past the point where you could argue that grading is subjective (e.g. busted corner and color-breaking creases on books described as NM). Despite the advertised free return shipping, they didn't cover the cost of getting the books back.
Despite my bad experience, I decided to use them again to pre-order a book that they were offering a good price on. They later ended up cancelling my order (because apparently they didn't bother to learn the ordering conditions for the book before setting their price), and offered me store credit for the purchase price. I didn't see their email at the time, so when I never got my order, I searched through my emails and found out what happened. So I had to waste my time getting a refund on an order that they never filled. I wonder how much money they keep from customers that never catch that. Bunch of thieves.