What changes hands then is not gold. It is the ordinary consumables people use every day and cannot improvise, held in small units so a trade can be small: batteries, fuel, coffee, medicine, soap, salt, tobacco. Buy only what your own household already uses, so that nothing is wasted if nothing happens.
And above all it is work. A skill does not run out, cannot be stolen, weighs nothing and is worth more the longer a situation lasts. Someone still has to fix the pump, treat the wound, put up the harvest and raise the next town on a radio. Each of the shelves below is one of those trades, and each is learnable now, from one book, in evenings.
The one skill that keeps producing after the stores stop. Nobody who can put up a harvest goes short of people to trade with.
Generators, pumps, chainsaws and mowers fail the same handful of ways, and almost nobody on a street can put one right.
Wounds, burns, infection and medication, at the point where the ambulance is hours away rather than minutes.
Finding it, moving it, catching it and making it safe. The first thing a household runs out of and the first thing it will trade for.
When the network is down, whoever can raise someone thirty miles away is the only news anyone has.
Shelter, rope, tools and the ordinary fixes that need no power at all. The oldest tradeable work there is.
What not to stock for trade: anything whose value depends on the situation getting far worse, anything you would not use yourself, and anything that makes your household the one worth visiting. A reputation for having plenty is the opposite of security.