Excerpts from the NPR post:
"... he guessed that the prices of the textbooks were going up and down with the college calendar. His theory was that prices would fall in summer (no one is looking for a nice textbook in July to curl up with on the beach) but then rise when classes begin and students really need the books."
"Over a year Bob and Kenny gathered data from Amazon on sales of all kinds of used textbooks, and they found the pattern was there for lots of textbooks — prices fell in the summer and jumped back up as classes started at the beginning of a semester. It was as if they'd found a stock that went up and down at very regular times, so that they could know exactly when to buy it, and exactly when to sell it."