Ships of the Desert. Those are what you see in the picture to the left. Yes, they are camels. Camels are called the ships of the desert because they were a lot  better equipped for the Saharan desert that the original horses that made up the first caravan. They could go days on end with out food or water, because of the fat storage in there humps, and there second stomach.
What are the items that traders can trade for salt? Well, there is along list of things such as, gold, furs, silks, fruits, cotton cloth, ceramics, tools, horses, slaves, and pretty much anything the ruler of the trader's land wanted. The slaves that could be bought were often prisoners of war that were captured from lands that were conquered by an African Kingdom.
What the West Africans supplied for the traders were, kola nuts, pepper, leather, and cotton. Traders traded these supplies from the west to the east, again, trading these things for things that the ruler of the African Kingdom wanted.