In case it helps - in the real world it goes like this:
The dealer must make sure that the customer is trustworthy, BEFOREHAND.
The dealer must make sure that he can deliver the promise, BEFOREHAND.
THEN, the customer may place a binding order with the dealer. Usually, the price quoted is locked-in for one hour, during which the customer must pay. In high-trust cases, the payment may come several days late. In low-trust cases, the customer pays first, and then the price is locked in.
At the lock-in moment, the dealer executes the trade with his suppliers. After that, the customer owns metal, not cash. If he fails to pay, the dealer is entitled to the damages. If customer wants to cancel, that is a new trade. If dealer wants to cancel, HE CANNOT. He MUST deliver. The customer has bought certain metal and he is entitled to receive it. End of story.
So, with reputable dealers, there may be problems, but they are not related to whether the customer has the right to his purchase. If that is denied, it is a scam.
This process is described in painstakingly detail in every country's legal code (should be, anyways).
When it comes to PMs, the customer also CANNOT cancel in Germany (revocation rights are revoked by law regarding PMs).
