We're not going back to barter are we?
I believe current theory is that barter was never really much of a thing and that some kind of fungible unit of account comes into play pretty much immediately. Though surely there's no way to know for sure.
Anyone who has done time in prison or jail knows that something quickly becomes the main unit of barter. Sometimes the guards will try to stamp out all trade, get rid of the current item of trade, something else rapidly takes it's place.
I was in jail once and they would give out these small cool-aid packets with dinner that you could mix with water. They were small, portable, fairly durable. 2 would generally buy someone's desert or 3 for a bland hamburger.