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That something needed to be small (so people could carry a number of them), valuable (so you didn't need 50 of them to equal a high-end item), and plentiful (so that you could get enough of them to trade for a high-end item) At times perfect gems were used, but even these were low enough in cost that it was hard to value high-end items. So the economy needed something that was more valuable to serve as a basis of comparison between the value of high-end items. Diablo 2 had a barter economy, but gold itself was not valuable enough to trade for any high-end items (because there was a max amount you could carry).
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