Procedure
To perform a silent trade, one group of traders would go to a specific location, leave their trading goods and then withdraw to a distance. Then play a drum to signal the other traders that a silent trade was taking place. The other group of traders would then approach and inspect the goods (most commonly salt or gold). If the goods met with approval, the second group would then take the goods, leave their own goods in return, and depart. This system of trading was used in particular in ancient Ghana. It was also used among the Kushites and the Aksumites.
It is described in detail in Robert A. Heinlein's science fiction novel Citizen of the Galaxy, where the crew of a starship stops on a world whose nonhuman inhabitants practice a form of silent trade.
The Greek historian Herodotus wrote about the gold trade with Ghana and Carthage: "The Carthaginians also tell us that they trade with a race of men who live in a part of Libya beyond the Pillars of Hercules. On reaching this country, they unload their goods, arrange them tidily along the beach, and then, returning to their boats, raise a smoke. Seeing the smoke, the natives come down to the beach, place on the ground a certain quantity of gold in exchange for the goods, and go off again to a distance. The Carthaginians then come ashore and take a look at the gold; and if they think it represents a fair price for their wares, they collect it and go away; if, on the other hand, it seems too little, they go back aboard and wait, and the natives come and add to the gold until they are satisfied. There is perfect honesty on both sides; the Carthaginians never touch the gold until it equals in value what they have offered for sale, and the natives never touch the goods until the gold has been taken away."
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