In herder society we have the rise of private property, and hereditary ownership passed through the linage of the father, (or patriarchy). That is the more significant prerequisite that must occur before any type of barter can occur. Private property had to exist first before there can be any type of barter. As to such a society "agreeing" that so many shells or lumps of metal are equal to so many cows, that is nonsense. Money was invented by the state, for enforcement of penalties.
I lived in Southern Africa for about 40 years and what you state here sounds like an over-complication. Sure, people had to have something that was of value to others and to desire something that others had in order to barter. That was and still is enough to barter. In pre-colonial days, most people even in "herder societies" did not own land; land was a communal asset which resulted in clashes when tribal chiefs gave the colonials permission to use communal land to graze their cattle; colonials thought that the permission was tantamount to handing over property rights.
The hunter gather San moved around in small clans depending on resources and clans met periodically to exchange goods (to barter). Again bartering was dependent on people having something that was of value to others and to desiring something that others had.
That was enough to barter and still is everywhere.
Bartering doesn't rely on preset valuations of goods/services being bartered. It often is just a matter of a gut feel assessment of value not unlike most of us agreeing to a price of most things when we purchase everyday goods and services.
