More and Better Barter
More advanced barter developed along ancient trade routes. In an article in Bravepages.com, Professor Leonard Lesko, chairman of the Department of Egyptology at Brown University, notes that southern Kush was a popular locale where Egyptians bartered gold, papyrus sheets and grains to obtain exotic items such as ivory, animal skins, livestock and spices. The barter system expanded and became a source of astonishing wealth for Egypt. Egyptians of the New Kingdom, according to the British Museum's website, traded precious stones, herbs, oils, horses and chariots to Africa and Asia in exchange for exotic animals, skins and minerals. As with the agrarian Egyptians, barter outdid coinage.
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http://classroom.synonym.com/bartering-ancient-egypt-19283.html
The History of Money - By Glyn Davies
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The ancient Egyptian economy, largely based on redistribution and reciprocity, set prices in units of value that referred directly to commodities. At first, for the purposes of exchange and trade, the Egyptians calculated the value of goods and services in units that were directly related to the necessities of life. Later, the calculation was made in terms of the weights of metals, such as copper or silver, though rarely did these metals ever change hands. Rather, their weight was used as a reference for value. For the most part, the ancient Egyptians never conceptualized the use of money. Definetly not the way we do today.
There are no tribal cultures on the east coast, and all the tribal cultures in the west lacked anything worth trading for, and caps are accepted currency in the entire post-war united states. Even The Legion and NCR still accept caps as money, even after making their own. So there is no "other currency" problem.
Speculation. We simply don't know it. Anyway, that's not the point.
Bethesda has shown a rather small part of the post apoc wasteland, however, there are for sure smaller communities here and there, we can assume that much. And it is simply NOT(!) likely that they all use the same fucking currency if there is no real conected economy, banking system and financial system with something to back it up. Even Fallout 1 went so far as to use water merchants in combination with caps. Having just caps, doesn't make much sense. You need something to back it up, even the ancient Egyptians used to hoard grain for that matter.
The smaller communities get, the more is bartering a common method of doing business as long it concerns the necessities of every day life, which doesn't mean that currencies never existest. There are historical examples of communities that had bartering as their main way of doing business, while other ones used a lot of currencies. The concept of money is simply to abstract given a realistic scenario, particularly the way how it works in our modern world. It doesn't get you trough the winter. It doesn't feed you or your stock. And it doesn't work for you.
Money or currencies work best in societies that have enough order and structure to support it. If we have to talk already about the basics of economy. By the way, I don't claim to be an expert. However, bartering existed for a long time NEXT to any currency and money, be it shells, copper, silver or gold. A currency, can only work as long as the people have the confidence in the currency that is used.
I will say this again, there is no way to assume that bartering doesnt happen in the setting of Fallout and that caps are the standard currency. Or that a currency is used everywhere the same way. Not when you have so many situtations where bartering is even used up to this day. Particularly in cases where people lose their confidence in money and currencies.
What ever if bartering makes sense from a gameplay point, is a whole different question.