You're talking about buying/selling, I am talking about what happens after that has taken place. After a supplier has agreed to sell at X price and a buyer has agreed to buy at X price they're no longer in an adversarial relationship, they're now in a symbiotic one.

How is it you think product physically moves from company A to company B? Through osmosis? There are physical trucks, and a ton of technological infrastructure behind the scenes to make that happen (from orders/invoices, through to inventory metadata (e.g. expiry dates), and all of the organisation that has to happen so all the trucks don't appear at the loading dock at the same/wrong time.