Maybe there's money to be made here in an automated way (call it the HFT of the used-book markets). I was looking for the Graphics Gems series on Amazon & Abebooks the other day, and the cheapest ones went for 1-4 dollars, while the expensive ones went for 40-60 dollars (not sure if they're getting sold at that price, but it's the asking price). If one would scan these sits and identify wide spreads, one could buy up the inventory at the bottom of the market and re-list at market price. Maybe one could deal with the transaction costs (S&H) by agreeing on bulk shipping or so, or letting sellers ship directly to the end-customer if one can be found within a week or two (agree with the seller that they keep the book in inventory for two weeks, if you find a seller within that time have it shipped directly, otherwise to your own storage).