Turning all serious for a moment, my best guess is that there are a number of second-hand book dealers whose business model is to buy up libraries at, say, 2p per book. As long as they can sell more than one in 2,500 for £50 a pop they've made money. It costs effectively nothing to put books up for sale on Amazon - a cheap barcode reader and a freebie ISBN database will do all the hard work for you, so there's no particular incentive to price sensibly. You could probably even write a programme which will automatically discount each book until you get a sale or take the price below some pre-determined level. In these days of increased recycling there's almost certainly a market for the books that go for pulping, too.