Not entirely related to technology, but I guess it could be if you're looking for tech textbooks!
I found a site a couple years ago (when i used to purchase textbooks) that aggregates all of the top bookselling companies and finds you the best deal on new and used textbooks.
I've managed to be able to find textbooks that were normally $200 for as low as $30 and purchased them. It works really well and just thought I'd share it for others.
Just enter a keyword, isbn or any info you have in the search bar and you're good to go