North Africans would come and leave their salt on the designated spot, normally a riverbank of some sort
Then they would walk away, far enough that they could not see who they were trading with. when they were that far away, they banged a drum symboling that the west africans could come to trade now.
When they heard the drum, they would come and leave the amount of gold they thought fit the amount of salt left. Then they would take the salt and leave, banging the drum when they got further away.
Then the North Africans than came back, and took the gold.
If they were satisfied with the amount of gold left, trade would continue.
Silent barter worked because both sides had it in their best interests to not cheat the other one. If they gave to little for the other amount that had been given, they would be cut off from trading.