Buying and selling books online (for students) usually turns out to be a much better deal than getting them from the local bookstores. It blows my mind how many people in my classes don't even consider online book purchasing as an option (at least of those I asked in my engineering classes)
Personally, I haven't been burned by any sellers yet.. Though I have been burned by usps (damn idiots need to leave notes when deliveries are dropped off at front office).
I still buy books at the bookstore until I receive the the ordered ones - the univ bookstores typically give at least 2 weeks return policy after the start of the term, and usually its a no questions asked return.
Other than that, I think i typically saved at least $50-100 each semester on books. Also, when end of semester time comes, getting 1/4 or 1/3 back vs full price (for those books that I have no further need for) from the local book stores bites... Sell 'em online instead. Even with half.com, amazon's, etc commisions, you still make out better than the univ bookstore.