The bookstore does what it can to provide the lowest retail prices available for its used and new textbooks. There are a few advantages to buying at the campus bookstore: 1) You can return a book easily if you don't need it or if the professor makes it optional 2) A percentage of the bookstore sales go back to the college 3) You're supporting local people who rely on the bookstore for their livelihood and whose salaries are directly affected by how high (or low) sales are.
Remember that buying online takes money away from people in your community. And don't forget--you end up paying taxes and shipping on some of these sites! That being said, here are some online resources:
Google has a search engine called Froogle (Froogle.com) that lets you compare prices on a wide range of consumer goods. It doesn't take long to run a semester’s worth of textbooks through to find the best price.
Amazon.com will almost definitely have all your textbooks available new, and many students sell their books used here after passing the final. If you're buying used, make sure you're comfortable with the level of wear. If the condition isn't described you may end up with a massively underlined copy. Also, make sure to get the right edition. Getting your books on Ebay.com will take a little more effort, but this can mean better prices. People sometimes sell things on Ebay without a clear idea of the value, and if you pick up on this and no one else does, you’ve got yourself a cheap book. Alibris.com is more of a specialty site. There's a lot of rare stuff here, but you can find some good deals and sometimes the best price for a book.
It may even be possible for you to get some of your course books for free. Book trading networks and online publishing projects are increasingly making this possible. The following article has a useful list of resources will show you How to Get Free Books for College.
Some of the stuff in the bookstore is a bit pricey. Pens and pencils sell for much more than they would in a grocery store, for example. However, remember that a percentage of sales will go back to the local economy. And don't forget convenience--you can ask the bookstore to order something if you don't see what you want. Finally, if you can’t get into town, the bookstore is always good for that quick energy drink and bag of gummi bears you'll need before class.