The sands of the Sahara Desert could've been a major obstacle to trade between Africa, Europe, and the East, but it was more like a sandy sea with ports of trade on either side. In the south were cities such as Timbuktu and Gao; in the north, cities such as Ghadames (in present-day Libya). From there goods traveled onto Europe, Arabia, India, and China.
Muslim traders from North Africa shipped goods across the Sahara using large camel caravans -- on average around a thousand camels, although there's a record which mentions caravans travelling between Egypt and Sudan that had 12,000 camels.
They brought in mainly luxury goods such as textiles, silks, beads, ceramics, ornamental weapons, and utensils. These were traded for gold, ivory, woods such as ebony, and agricultural products such as kola nuts (which act as a stimulant as they contain caffeine). They also brought their religion, Islam, which spread along the trade routes.
Nomads living in the Sahara traded salt, meat, and their knowledge as guides for cloth, gold, cereal, and slaves.
Until the discovery of the the Americas, Mali was the principal producer of gold. African ivory was also sought after because it's softer than that from Indian elephants and therefore easier to carve. Slaves were wanted by the courts of Arab and Berber princes as servants, concubines, soldiers, and agricultural laborers.
When Sonni Ali, the ruler of the Songhai Empire, which was situated to the east along the curve of the Niger River, conquered Mali in 1462, he set about developing both his own capital, Gao, and the main centers of Mali, Timbuktu and Jenne, into major cities which controlled a great deal of trade in the region.