The absolute cheapest way to obtain your books: Choose a college that includes textbook rental in the price of tuition. Here in NC I am aware of only four schools that use this system: Two of them are large schools in the UNC-system. Another is one of the UNC-system's lesser-known schools. The last is a very expensive private school, but since the cost of attendance at that schools is approximately 3.5Xs the cost of the state schools, this is a questionable way to avoid buying books.
She goes to the bookstore, obtains a print-out of her schedule, takes it to the book counter, where the worker bees say, "One moment, please", and they bring her a stack of expensive books. She signs for them, and they're hers 'til the last day of exams. If she fails to return them by the last date, she's billed for them. I went with her last time she picked up books, and when they gave her the books, she commented that one book had water damage -- they brought her a new book.
I think it's a fantastic system. It says to students, "Books are so important that we're going to make sure you have them. You're not going to have to chase all over the internet so you can have what you need to be successful in college." And on multiple levels, it just makes sense for the college to use the same books over and over.
My daughter's a sophomore, and her total semester book costs have been $25, $10 and $15 -- all for consumable lab manuals.
The biggest shocker: Her Chemistry book alone would've cost $360, had she bought it new.
I did buy her several textbooks while she was in high school taking AP classes, and this is what she and I learned together: Pay attention to the ISBN number. Say it's 123456789. Type into Google, "Buy 123456789" and it'll pop up all the sites that're selling it. Do the same when you're ready to sell. Prices vary widely: I remember the first book we ever bought for her was something like $12 on one site, $108 on another. It was an eye opener for me, but she was downright shocked.