The Bookstore has arguably the best selection of Badger attire/memorabilia in the State Street neighborhood. There's a mix of higher-end athletic brands (think Under Armour, Adidas, etc) with really affordable, collectible clothes. The sales can be pretty good, too. Check their website for coupons before heading down there, there's often a coupon for clothing items that you can print and use for a small discount.
As for textbooks - in my experience, the Bookstore has never been a particular bargain destination. They're not particularly well-known for great buy-back offers, either. Part of the textbook business, I suppose. It's very easy to find cheaper textbooks from online sellers or even other bookstores on State Street. Look elsewhere before you buy if you're interested in saving money.