You can get some great bargains as far as textbooks are concerned online but it is very important that you remember a few simple rules.
1: Check the title and author are the same as the book you wanted. Sounds simple but as a librarian and book-seller I have had people request / buy books and be quite definite about Title and Author only to have completely the wrong item arrive.
2: Check the edition number, the last thing you want is a book that is terribly out of date. E.g. how useful is a computer book which deals with writing programs under DOS or a physics text that has no reference to Quantum theory?
3: Check that binding is in good condition. If the photo / Description is unclear ASK the seller to describe the book's condition to you. Are there marks on the pages? Will they interfere with your study? Are there high-lights on the page? They will prevent photo-copying.
4: Finally check the postage, make sure you are not getting a 'bargain' text with an outrageous postage. If you are unsure ask the seller to use the cheapest mode of postage possible and remember that this will slow down your items arrival.
If you follow these 4 simple rules you should be relatively certain of getting what you want, at a price you can afford & when you want it.