Wikipedia
Silent trade, also called silent barter, dumb barter ("dumb" here used in its old meaning of "mute"), or depot trade, is a method by which traders who cannot speak each other's language can trade without talking. Group A would leave trade goods in a prominent position and signal, by gong, fire, or drum for example, that they had left goods. Group B would then arrive at the spot, examine the goods and deposit their trade goods or money that they wanted to exchange and withdraw. Group A would then return and either accept the trade by taking the goods from Group B or withdraw again leaving Group B to add to or change out items to create an equal value. The trade ends when Group A accepts Group B's offer and removes the offered goods leaving Group B to remove the original goods.
This system was used in many parts of ancient Africa. Silent trading was mainly used during the period 500 to 1500. The practice was also well established between tribes in Africa in their trade with India. Cosmas Indicopleustes describes this practiced in Azania, where officials from Axum traded for gold with beef. Prince Henry the Navigator of Portugal recorded this practice when he occupied Ceuta in 1415.
Also in West Africa,gold mined south of the Sahel was traded, pound for pound, for salt mined in the desert. The salt from the desert was needed by the people of Sahel to flavor and preserve their food and the gold had obvious value, especially in trading with the European people. Because of this trade, cities grew and flourished and parts of West Africa became commercial centers. West Africa produced large amounts of gold until about 1500 AD. The communication in this gold-for-salt was carried out using drums.
Silent trade might be used because of an inability to speak the other traders' language, or to protect the secrets of where the valuable gold and salt came from.
Silent Bartering has been used since ancient times, such as the ancient Ghana Empire. The Ghanaian salt traders would leave pounds of salt by the Niger river and the gold traders would leave a fair amount of gold in turn.