If they are rich enough to have enough gold at home to "slam" on a grocery counter then they will be smart enough to take their "bar" and use it to make a long term contract with a party that can supply the needs they seek over a long period of time.
If the economy collapses how will the grocer arrange to get food in the long term?
How will a grocer in New Hampshire get a delivery of frozen food produced in Ohio? How will the factory in Ohio pay employees to do the work if the economy collapses (which means the dollar becomes worthless)and there is no currency to pay employees? how will they get diesel to put in the tanks of the trucks to haul the food?