The bookstore is currently buying books back. The sooner you sell your books back, the better the price. The last day to get the "best price" is May 18th. It can't hurt to go over and ask how much they are buying your books back for.
A lot of students wait to do this, and as a result get very low prices for the textbooks. Going in now will lead to you getting the best price - for example, the CIS 101 textbooks will apparently give you $48.25 back.
Of course, wait until after you finish your finals for a class before you sell the books back!
