From what I have found, Abebooks, Valorebooks, Ecampus, and Amazon are all about the same in terms of price and reliability, but search all 5 directly to find best bargain. If you have more advice please comment so we don't have to repost on Facebook again.
FREE shipping over $59 which can add up if you are buying couple different books, PROFESSOR - $3 off orders over $75
Best deals are on used books usually, but sometimes Amazon main page actually has better deal, they also have cheap electronic copies. They also have great buy back program come end of semester...
FREE two day shipping and movie streaming. Get 6months free with .edu email address just set reminder to cancel else get charged $39 I think for 1year (But still thats 50% off full price prime membership and if you are buying a lot or want movies this is good deal. CLICK HERE
(Seems to have some of cheapest books, cheap renting too.)
Very similar to abebooks. Good coupons on retailmenot. (Good Renting)