Although I have since revised my ill-informed freshman year belief that the University Bookstore is evil and seeks to do all students harm, I can't emphasize enough how much money you can save by seeking out low-cost alternatives. One option that I regrettably never tried out is the ASM book swap, which is taking place next Monday and Tuesday.
If you have any books you're looking to sell, you take them in on Monday, between 11 a.m. and 7 p.m. to Room 116 of Memorial Library. The pricing is determined, and then the following days, on Tuesday and Wednesday, you can go in and browse for books to buy. Any books that you provided that are not sold will be given back to you on Thursday.