Hi Mark, I suggest you keep it real simple at first. Do they sell a limited selection of new books, or an ever changing selection of used books?
You said they already have a website and I assume it either has individual pages for their limited selection of books, or some sort of list or table of their used books.
If they have the limited selection, I suggest using UltraCart since they provide the ability to put BUY buttons and VIEW CART buttons on any existing html website - AND they have an affordable full featured checkout system. So all you would need to do is create the book items in your UltraCart catalog, then copy and paste the button code where you want it on their current website. It is pretty easy to customize the cart to make it look like their current website. If their sales volume is low, just use Paypal as the payment processor and skip all the expense and hassle of a credit card merchant account. I use UltraCart for my two stores. Just search for easydigging or bestdryingrack if you want to see the cart in action. My carts are pretty heavily modified, so see the UltraCart site for more stock examples.
If they are a used book store, it may be easist to just use Paypal's system completely and just generate a new button each time they list another used book on the site. The used book route would be quite a maintenance chore. Best to see if Paypal has "single use" Buy buttons since we assume there is only one copy of each used book...