The Wangarans would leave an amount of gold dust by what they wanted to buy, bang a drum and leave.
The North Africans would hear the drum from the riverside and begin to come back from their place of waiting.
The North Africans would look at the amount of gold dust left by the Wangarans and decide whether it was enough, in the case of in not being enough they would leave everything as it was and band the drum and return to thier waiting place
The North Africans go back to their waiting spot and wait for the drum to signal proper payment.
When happy with their payment the North Africans would take the gold dust , bang a drum and leave the wangarans their bought goods.
The silent trade did come with a few large advantages, one of which is that the silent trade allowed trade to take place with people who had different languages and without risk of conflict. One other is that this form of trade allowed the Wangarans to keep the location of their mines well hidden.