Trade in the Aztec empire was mostly barter, but I'm surprised to read that a few commodities were plentiful, portable, durable, interchangeable, and stable enough to serve as money. “Cacao seems to have been the most common form of money, and it did, indeed, grow on trees. It was widely accepted as payment for both merchandise and labor.” There were even, this book claims, cacao counterfeiters.
The merchants of the Aztec empire had a guild which set its own laws, enforced them, judged and even had some control over prices. Merchants “entered enemy territory as spies, they could declare and engage in wars, and they could conquer communities.” They became very, very rich.
What struck me about the Aztec merchants, though, was what they ultimately used their wealth for. The vast majority they spent on lavish feasts for other merchants. The guests gorged on tamales of corn and chile, drank chocolate, and ate hallucinogenic mushrooms. Throwing a feast was how you converted riches into an even more valuable currency: social status. “Through this elaborate system of potlatch-like feasts, the merchants were obligated by the guilds to use their accumulated wealth for socially prescribed ends.”
The book is The Aztecs of Central Mexico: An Imperial Society, by Frances F. Berdan.