This is what i do.
Look book up at school website obviously overpriced. Look book up on both Amazon and Barnes and Noble and make sure ISBN match correctly also compare the price. Then I check the book on half.com
I only buy books on Half.com if they have a big price difference because you still have to pay for shipping and sometimes you are buying a used book and multiple books equals several shipping chargers.
While if the book has almost no price difference or shipping will almost equal the savings buy from amazon or B&N because they provide free shipping.