Reference the books you need and write down their title, author, ISBN and edition. Use this information and a computer with Internet access to find similar books on auction websites to get the market value of the book currently (this gives you a benchmark quote).
Check your pricing offered at your local school bookstore for buying your books used from them. Write this information down when they give you a quote. Compare this with the market information. Do not buy your targeted books yet.
Go back onto the Internet with your computer and access. Run a search for "buying used text books" through a website search engine. Check your book values offered by these companies by inputting your ISBN and book information into their website price check page.
Write down the prices offered by the websites you visit. Compare the Internet prices offered with the bookstore price versus you earlier market research on auction sites. Buy your books from the seller with the lowest price offer including any fees and shipping costs involved.