Do they do what they're supposed to do?
Do they not do what they're not supposed to do?
Realistically, beyond that, we end up making a lot of assumptions about what you know and what you're looking for. That having been said, what do you think "are the things to be taken care of when we test order entry applications"?
Take your example, and look at all aspects of the book ordering process at amazon. Explore options, except for making multiple purchases, of course. I believe that with a little effort you will have hands-on experience testing an order-entry system as well as a book that explains the theory behind it. Congrats, you're on your way.