In the case of Trans-Saharan trade this is little more complicated.
The principal trading articles were: gold, slaves and salt. This is what opened the trade in the 8th-9th century. The salt originated from central parts of Sahara, the gold and slaves from the south. The caravans were operated by the Berbers... originaly the middle-men of this trade. They had the salt and they were changing it for gold... that gold they transported to the north where they could buy goods for themselves for that gold. As the volume of trade increased, the northern goods started to be sold in the Sahel too, but still the most valuable trade good there was the salt (and to a degree also horses). The ones profitting the most was therefore the northern part of this trade complex - centers like Sijilmasa, Tahert/Tlemcen, Fezzan and Tunis getting cheap gold.... those were as trading centres totally dependent on resources comming across the Sahara, not the other way around. From this perspective the main direction of the trade was undoubtedly form the Sahel's gold fields northwards.
The only thing to effectively counter this direction would be the technology (and religion), which obviously spread the other way around (OTOH, the civilizations of the Sahel were not that much technologically backward so that the technologies comming from the north would be so essential for them.
That said the starting points of these trade routes should IMHO be Bambuk, Bure and Jenne in West Africa (Jenne as by far the most important trading center of the region before the rise of Timbuktu in the 14th century) and then Kano and Kanem/Bornu area in the central Sahara (as the principal sources for slaves).
If you want little better map also with the main trade goods of Trans-Saharan trade, here you go: