No, you have not chosen the most relevant rule of law.  Remember, the issue in this case is:

In the absence of an agreed time for payment, when must a buyer of goods pay the purchase price?

The rule of law you selected is:

(a) A contract for the purchase and sale of goods is legally binding and can be enforced in the courts.

This rule does not provide an answer to the question raised. 

The rule that provides the answer to the question raised is: (c) Delivery of goods bought and sold, and payment for those goods, should take place at the same time unless otherwise agreed.

In more complex case studies, more than a single rule of law may be involved, but the principles of selecting them remains the same.