They're not 2nd hand or seconds, and it's not a club that you commit to.
I've bought loads of books from them in the past and the quality is great.
Sometimes it's remaindered stock from a publisher, but often it's just a very special deal from the publisher.
I used to work in publishing, and it was great to get a title in a book club, because it meant you could do a far, far greater printrun, thus making the unit cost of your whole run much cheaper. Often you didn't make a great deal on the book club deal, but you increased your margin on every other book you sell iyswim.
Also Book People have their own publishing house Ted Sharp, and sometimes they actually do their own print run. (I don't know how this is handled financially, but they must use original artwork, as the only difference is the logo on the cover, barcode, and the title verso (one of the early pages with publishing info.)
Anyway, again, the quality is absolutely fine and many people don't even notice that some books have a different publisher name on them.
Go for it!
what are you buying?