Here are a few things to take into account when purchasing books online. I have had excellent experiences with both half.com and Amazon.
Used Versus Unused
In general, it is best to buy most of your books used. You probably already know this, which is why you are buying from half.com instead of your campus bookstore. However, keep in mind that "Used" gives no further description than the fact that the item was, at one point in the past, purchased as new. Sometimes you can find better deals on new books than on used books (I've had this happen more than once). Shop multiple sites - using both Half.com and Amazon is a pretty safe bet.
Things to Look for- A good user reputation - read buyer comments and check ratings carefully. User experience is also important to notice, though many new users are still good sellers
- A quality above "mediocre / poor": If you want to sell this book back to anybody, ever, you might consider getting a book of at least decent or good quality.
- Legitimacy: How much less expensive is the book than other sellers? If it's $5, cool. If it's $40 cheaper, be careful.
- Quality: Is it paperback or hardcover? Read comments - very good to one person might not mean the same to you.
- Edition: Is the book the correct edition for your class? Email your professor if you can't find it anywhere, or search by the author's name to find newer editions. Purchasing an old edition may (very occasionally) result in missing chapters or problems. Textbook companies do this on purpose. Nice profesors sometimes offer problem numbers for new and old editions
Beware the International Edition
International editions are generally paperback copies of books. Though many are in good quality, they often lack answer keys and may even be missing chapters. Many students I know have not had good experiences. They are also practically impossible to sell back.
Used books are great, but use your instinct. If it seems extremely sketchy, it very probably is.