Not directly on Kat's point, but perhaps of interest to potential "used" book buyers, and following on what Beer describes above--

A "used" book seller (via Amazon) answered my question about how she can afford to let books go so cheap-- sometimes it's a penny a book, plus shipping. She said that books going at that rate have been bought by the case at auction, unexamined, and may be new or used, some new with slight water damage suspected from box damage, and that there are enough "good" items in these cases bought at auction that in total, she makes out quite well. She's happy to let the penny books go so cheap because it reduces her outlay costs as long as the shipping is covered. I've bought several books this way much cheaper than their "new" price and been surprised to get brand-new books with tight spines showing they were never opened.

~Susan