A lot of Penn professors (in the liberal and social sciences) avoid ordering their books from the Official Barnes and Nobles bookstore and order their books here at the Penn Book Center. Why? Because they believe they are getting students a discount. The prices found at this store, however, are not cheaper. Moreover, they only order a limited quantity, so if you are not first in line, its likely you will not get the books for your class in time for the first class's reading.
Aside from textbooks, this store also sells books to the public. Their selection is small and their prices are average. The best reason to go to Penn Book Center is for their sidewalk sales. Every so often, they'll put some tables outside with bins full of books and sell random gems for as low as $0.99. So, if you are on your way to the law school or about to grab a bubble tea, stop by Penn Book Center and check out their sales bin!
Also, beware, they are VERY strict on their return policies.