For those unfamiliar with the term, "remaindered," it refers to a strategy for unloading obsolete inventory. But let me back up. You know (from previous posts) that standard selling terms in the publishing industry allow retailers to return unsold books for full credit. As I understand it (if anyone knows a different story or greater detail, please post a comment), Simon & Schuster ran a post-Depression era promotion offering to allow booksellers to return any books they did not sell. It worked. And it took root. Other publishers followed suit and 80 years later we are still doing it (and paying for it).
Sometimes we get more books on the shelves than will "sell through." For example, if a retailer orders 12 copies of a book and only sells 2 copies in the first 60 or 90 days, they'll return 9 copies and keep one copy spine-out on the shelf. When those books come back, we put the clean ones back into inventory and write off the damaged ones. As demand naturally tapers in the life cycle of any book and the publisher is left with inventory collecting dust in the warehouse, storing the books may cost more than the revenue they will produce. When supply and demand reach that point, remaindering becomes a viable option.
Remaindering books is when a publisher sells books in lots for pennies on the dollar as a means of getting out from under obsolete inventory. These are the books that end up in the dollar stores and bargain bins.