Who says it would have to be a grocer? If it was me I would deal with a private farm or ranch to supply me with food, then contract with others to supply other needs.
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?
Your original scenario was how would a gold owner eat. If the economy collapses, then factory workers will be on their own.
I live in a town of 6 thousand people. There are maybe 3 commercial farms big enough locally that can produce more food than they need to eat for a year. FDA tells us the average commercial farmer can produce enough food to feed 125 people for a year.
I will be generous sand say they can produce 3 times higher than the average. And that is of course when they use gas powered machinery to farm. (We will assume the farmer has his own personal oil well and refinery and a perpetual motion machine to run it so he can produce his own fuel to run his farm machinery on)
So our local commercial farms can produce enough food in the year after the economy collapses to feed 1200 people. What will the farmer do with your gold bar when a couple thousand hungry people show up at his farm wanting food? Do you think he has enough guns and ammo to chase them away? If he doesn't does he then give them food or risk dying at the hands of a hungry mob. If he decides to give the food out then what is your long term deal worth?
Do they just lay down and die? Or do they go to the local farms and make an offer the farmer can't refuse?