I am a bit dated, it's been the better part of a decade since I had to buy textbooks, but by and large the price differential from ebay/amazon for used compared to the campus store wasn't that massive for books beyond the lower-level classes and electives, especially once shipping was included. I just checked several of the standard physics senior and grad texts, and though definetly cheaper new at amazon, the used wasn't much better than I'd expect at the campus store, including S+H I'd say they might even have been more. Higher level texts just don't have many copies in print and their owneres tend not to sell them. A single freshmen level course might spew back hundreds of it's text post-finals at a single university while a post-baclaurette text might only have 10,000 or so copies in print, 99% of which are either fresh from the printer or sitting on someone's shelf who simply has no intention of selling it. I'd imagine genuine bargain hunting sites might give amazing prices, but I'd be surprised if they had any copies to sell, I was very surprised to see a few dozen copies of Jackson's Classic Electrodynamics (a second year grad physics text) avaliable for sale on Amazon, and the cheapest of those was only 25% below their price for a new one, which is fairly parallel to used at a campus. So while I certainly encourage bargain hunting, I wouldn't expect to see many non-intro texts up for mega-cheap prices anywhere except at a fluke, I hope I'm wrong