When buying textbooks, the library bookstore is always really expensive. Try buying from upperclassmen first, or from other websites. Some books can only be bought from the bookstore though or from students if they’re a “custom” Stony Brook edition. If you have facebook, a lot of people use it to advertise their textbooks on groups with Stony Brook students.
Make sure when buying from other students that its the right edition. I know that MAT 123 changed their textbook completely (not even a new edition, but a new book entirely) so you’ll have to buy it from the Stony Brook store or an outside source. Even Summer Session MAT 123 students used the old book, so don’t get tricked. EST also went from 11th edition to 12th, so anyone trying to sell an EST book from this summer or previous doesn’t have the right edition.