check out this thread for websites for textbooks
Used/Cheap Book Websites
i found campusi.com really helpful as it could give me a quick comparison of books and websites.
one thing to keep in mind is the seller's reputation, especially if you are buying books on ebay. the better the feedback, the less likely that your book will "get lost in transit". usually amazon or sellers on amazon are very good. i live in canada and most of my books have come from american sellers, and the time for shipment pretty much varies. the fastest for me was 1 week, the slowest was like 3-4 weeks. also, don't forget to factor in the shipment price when ordering books, because the book could be for 9.99, and the shipment could be for 24.00