The slave trade was very profitable for slave traders. By the late 18th century, a strong male slave could be sold for about $600 to $1500 (which is about $9,000 to $15,000 in United States dollars today). Because of this, kidnapping people in Africa and selling them into slavery became more and more popular. Historians think that African warlords, kings, and private kidnappers - Europeans, Americans, and Africans alike - all participated in kidnapping people into slavery.