I have got the same assignment, started working on it since last couple of weeks. I totally agree that the check availability of a product should be queried from big smokes inventory system and not from manufacturer inventory systems. Check availability use case clearly says that the inventory system sends response with the number of units(given product) available for sale. So if we assume that the customer should enter the quantity within these limits i.e., les than or equal to the number of units available for sale.
I have a question here - Check availability of a product is queried when the customer selects a product in search catelog and adding a product to the shopping cart. As per the use case, the checkout use case does not check the availablity of a product and the customer can modify the quantity. When the customer modifies the quantity during checkout, Should the system check the quantity against the inventory system. What is/was your assumption? Please correct me if my assumption does not make any sense..
BTW, Did you already submitted the assignment or still working on it?
Thanks in advance,