(b) That's right. The parties to a contract of sale are free to agree when payment is to be made - before delivery, at the time of delivery, or after delivery. But if they do not discuss and agree on this matter, the relevant sale of goods legislation requires payment and delivery to happen at the same time - the obligation to pay is reciprocal to the obligation to deliver, and the buyer is not entitled to delivery without payment. See a list of the relevant sections.

In the example, since no other time for payment was agreed, A must pay B when the racks are delivered. His failure to do so is a breach of contract.