Buying Books Online
Many students wish to buy books online to save money. There are advantages and disadvantages to doing so. Remember that when you buy books from an online vendor rather than The Bookworm or Books That Matter, you will have to pay shipping and handling; you may not get immediate delivery; and you will have to give a credit card number out over the Internet. Beware! If you must buy over the Internet, I recommend using a payment intermediate service like www.paypal.com, which will let you pay directly from your credit card or bank account without giving your credit card number out over the Internet.
If you must order books online, make sure to match the ISBN numbers exactly. This is the only way to guarantee that you are getting the right edition of the book for your course.
I also recommend that you pay for premium (first-class) shipping rather than "media mail," which can take up to three weeks to be delivered.
Having given you that warning, here are some online used book services that I have used with little or no problem. (That statement is not an endorsement of any of these sites; I'm providing the links for your information only.)