Trans-Saharan trade was when people traveled across the Sahara to reach sub-Saharan Africa from the North African coast, Europe, or the Levant. The trade existed during the prehistoric times. The trade became popular from the 8th century until the late 16th century. The trade was also used to transport slaves and food to different places. They also traded a lot of gold for salt, and salt for gold. The center of the trans-Sahara trade was Ghana. The king of Ghana taxed everyone who came into the country to trade. He was considered to be the richest man in the world.