No, it's often quite possible to scan the watch while the transaction is still open (depends on the store; not all of them support that). The payment method is still open, and is closed by the cashier.

Yeah, it's "trusting" the store, but it has never resulted in unwanted charges. I do it for the person in line behind me. I can afford it, if there was to be an issue, and the service desk is about ten feet away.

In Japan, they make a ceremony of giving you your goods before accepting payment.