Merchants could seize the opportunity here - if your store is full of resellers rather than the public, it's time to up YOUR game and start charging more for your used books.
The way to prevent resellers from picking through your inventory for arbitrage is to correctly price your books from the start. With accurate pricing you can still undercut resellers and not make each book a commodity -- Resellers incur a multitude of costs: time, shipping, 15% amazon fee, etc. If you raise your prices to resaleValueOfBook - resellerCharges + $0.01, you cut out the combing by resellers and still offer a sizable discount from Amazon.com