Due to their lifestyle, nomads had a special relationship with trade. Nomads are people who do not settle and build cities in one place, but continuously travel. While other societies were able to stay put in one city, and generally figure out how to supply for all or most of their needs in these cities (agriculture, metallurgy, etc), nomads did not have this luxury.
Think of the nomadic people of Central Asia or the caravans of Arabia. These people traveled constantly, often having to find new sources of food and shelter. People with this lifestyle had to be good at trading. It was essential to their survival to figure out what people in various regions needed and were willing to trade for, and to supply those things when they arrived there. Nomads were more reliant on trade due to the nature of their lifestyle, and better at it for the same reason.
They also had the advantage of coming into contact with a diverse range of people, with different products and customs, and were able to serve as middle-men between these societies, enjoying the profits of playing such a role.
