When two people each have items the other wants, both people can determine the values of the items and provide the amount that results in an optimal allocation of resources. Therefore, if an individual has 20 pounds of rice that he values at $10, he can exchange it with another individual who needs rice and who has something that the individual wants that's valued at $10. A person can also exchange an item for something that the individual does not need because there is a ready market to dispose of that item.
David Graeber argues that the inefficiency of barter in archaic society has been used by economists since Adam Smith to explain the emergence of money, the economy, and hence the discipline of economics itself. "Economists of the contemporary orthodoxy... propose an evolutionary development of economies which places barter, as a 'natural' human characteristic, at the most primitive stage, to be superseded by monetary exchange as soon as people become aware of the latter's greater efficiency." However, extensive investigation by anthropologists like Graeber has since then established that "No example of a barter economy, pure and simple, has ever been described, let alone the emergence from it of money; all available ethnography suggests that there never has been such a thing. But there are economies today which are nevertheless dominated by barter."
Even before visiting the Marquesas, I had heard from men who had touched at the group on former voyages some revolting stories in connection with these savages; and fresh in my remembrance was the adventure of the master of the Katherine, who only a few months previous, imprudently venturing into this bay in an armed boat for the purpose of barter, was seized by the natives, carried back a little distance into their valley, and was only saved from a cruel death by the intervention of a young girl, who facilitated his escape by night along the beach to Nukuheva.