Trading benefited people by helping people get things they needed to live that they couldn't get themselves
I am blue person and I'll be going through desert to get water from well. Then I go trade salt for gold.
People traded by leaving cloths with goods on them and beat a drum and walked off. people would then place gold dust next to the goods and beat the drum again. Traders would then take the gold and leave. If it wasn't a good deal they would leave the gold and goods to wait for better offer.