Prepare an REA model for the sales process from the following amended version of the Horizon Books case. Horizon Books is a bookstore in downtown Philadelphia. It carries an inventory of approximately 5,000 books. Customers come in and browse the shelves, select their books, and take them to the cashier. At the cashier's desk they can also discover whether a particular book is in stock, place orders for books not currently avail- able in the bookstore, and collect and pay for books previously ordered. The cashier has a book database that is consulted for every query. There are no credit sales. All customers pay for their purchases at the time of purchase.