Purchasing the book through your campus representative is cheaper. We sell the book to them with a discount and they pass the savings down to you. By buying from them you also won't have to pay for shipping!
We give a 10$ discount to the local representative and they usually choose to offer the book at 5$ less than the internet price + you save 6$ on the shipping so overall you will save 11$ if you buy from them.
By keeping 5$ in your pocket, and putting 5$ in your local representative's pocket, one part of the profits from this book will stay in your university.
Stay local.