Even if it weren't a terrible, awful idea from a democratic point of view, it's just silly from a logistic point of view. This means you create a single point of failure for your entire society: electricity provision. Got a power shot down? Suddenly nobody can buy anything. Got a war on your territory? Your trade routes will get destroyed 4 times faster. Want to be a reserve currency? Guess what, now you have to do l…
Most stores aren't going to be able to sell you anything with no power, regardless of taking card or cash, because sales need to go into a database via a register, update inventory, etc. Sure a few small family run businesses, maybe restaurants, might be able to take cash, but almost no chains will be able to run and that's most stores in most places.
After a few days, I guarantee even the big ones will manage the inventory with pen and paper, with the blessing of the boss and the gov.
But even without all that, having the ability to have a black market in a time of crisis is priceless.