I personally used www.alibris.com
this past semester. Not sure how they were price-wise, I know they were quite a bit cheaper than the on-campus bookstore, so thats all that mattered to me. Almost every single book I ordered came quickly except for one, which I just got today. If we hadnt cancelled classes the first week, I wouldve now just finished my second week without a book.
Alibris doesnt sell many books, mainly they network together booksellers. I did a group project on them last semester, so I decided to use them this semester.