bigwords will query all major resellers, including amazon, ebay, textbooksx, etc.
i run into my campus bookstore, jot down the isbn's from the required books, jot down their new/used prices, and plug it into bigwords. sometimes with tax + shipping, its worth it to buy it locally. i'd say 90% of my books were purchased online, though.
and bookmark that site for the end of semester. i just sold back all of my books through bigwords (except math + chem--i hang onto those by choice) for about an 80% recoup of expenses.
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I have all of my books but next time Im going to look on there first. I can get everything for my books online because they have the book orders online at Bowling Green.