If a new e-textbook is under $15, what's the point of seeking a used one? Even if that were an option, you wouldn't save much, and I would expect e-textbooks to be updated more frequently, so you might be missing something if you bought last year's e-textbook.

I suspect with lots of students they sell their old textbooks to save space as much as to recoup some of the cost (or get some money for beer).