An interesting arbitrage opportunity

After dining on Boston clam chowder at Panera Bread this afternoon, I went across the street to Ram Book and Supply and offered to sell back my books from the pricing and sales classes. The cashier only offered $20 for the pricing book but offered an entire $9 for the sales book. It just happened that I bought the sales book brand new on eBay for under $7 (including shipping). So needless to say, not only did I fork the book over I will be returning to eBay to see if there are any more such deals.