Furthermore, private money has a long history which actually preceded coins from kings. Call them tokens, good-fors, wooden nickels, chits, scrip, they have carried trade and commerce time and again. And they are working now. Indirect Trade Excavations of Ice Age tombs reveal amber, jade, and shells, other objects of no practical use, hundreds of miles from their places of origin. They could have only been carried as tokens to be used in indirect trade.
Indirect trade is an abstraction from barter.
The stone ages ended about 8000 BC when copper was discovered. Cuneiform records from Mesopotamia at about 2400 BC tell of silver being weighed out in payment. This was indirect trade in a material that, like amber and cowrie shells, had no immmediate use. Silver and gold are pretty, but not suited for agricultural or hunting tools. By 700 BC, merchants were die-punching nuggets of electrum to mark them so they wouldn't have to be reweighed.