It marked the ancient border with Nubia to the south and was the conduit for the camel caravan trade in African products such as gold, ivory, and spices.
Africans did not trade gold. 
mnli hypothesis: Africans did not trade gold.  premise: It marked the ancient border with Nubia to the south and was the conduit for the camel caravan trade in African products such as gold, ivory, and spices.