I read on NPR a while ago some guys found an arbitrage opportunity in book prices. So - he would track the most sought after books, buy them when price were low, usually around July/August, and then sell them back on Amazon when prices were high, around the time of September and January. Makes sense.
http://www.npr.org/blogs/money/2014/11/10/363103753/textbook...
(best imo was talking about the whole mess around writing textbooks)