I've both bought and sold textbooks on eBay. There is a lot of crap to sort through, as eBay is inundated with International Edition textbooks. The International Editions are often slightly off with their page numbers, and have zero resale value. Still, a fair number of students sell either new or used books on eBay at excellent prices. When I sell, I make sure to always list my BIN price lower than Amazon used.
As for e-books, that would never work for me. I need to be able to turn the page, highlight stuff, flip back and forth, etc. I know most of this is possible with e-books, but it just isn't the same.