Trade beads (sometimes called aggry and slave beads) were otherwise decorative glass beads used between the 16th and 20th century as a token money to exchange for goods, services and slaves (hence the name), this included the African slave trade, slavery has existed in Africa for how long?
Trade beads:  This included African slave trade. Slavery in Africa: Slavery has existed in Africa for many centuries, and still continues in the current day in some countries.
many centuries