The purchase circles for universities are in many cases probably heavily based on the assigned reading for popular courses. Amazon and various text-swap websites (see http://www.textswap.com as an example) are popular alternatives to outrageous college bookstore textbook prices. In my experience, an increasing number of faculty members have started to recommend alternative book sellers, and are more amenable to making accomodations for students with older editions of a particular textbook (hand-me-downs, bought used for dirt cheap, etc).
Anecdotally, two observations: I've saved a significant chunk of change by buying textbooks online, where possible; and the type of student who buys books online is also usually the student who is paying his own way or racking up massive debt for school, whereas the student who buys from the college bookstore is probably on his parents' dime. So you'd likely get a snapshot of a particular economic stratum by checking the Amazon P.C.s, rather than a broad overview of what college kids are reading.
Quickly checking the purchase circle for the University of Pennsylvania, I find at least five books in the top ten that are clearly assigned readings, and three books about making megabucks quickly and easily:
http://www.amazon.com/exec/obidos/tg/cm/browse-communities/-/215100
-- Matthew Snyder Philadelphia, PA