How to Solve an Impossible Problem

Here is another story from ‘The Shadow of the Sun’ by Ryszard Kapuscinski  (one of the best books I have read, beautifully written).

It concerns a rather improbable method of trading that used to take place between the Tuareg people of the Sahara and the Bantu people of the Sahel.  For centuries a great enmity has existed between them, villages burnt, people killed and enslaved - made worse in times of drought.  Both groups had valuable commodities to trade, the Tuareg salt, and the Bantu gold.  But such was the level of fear between them, it was impossible to see how they could trade. 

 How did they solve that seemingly impossible problem?   You would assume that they simply traded through a third party, but for some reason, this is not so.

If fact, they contrived an extraordinary solution. 

When the Tuareg arrived at the trading location, they left salt in orderly piles.  They then retreated half a day’s travel.  The Bantu duly approached and left a measure of gold by each pile of salt.  After they had retreated, the Tuareg returned and if they deemed the amount of gold to be sufficient, they took it and left the salt, if not, they left both the gold and salt in place and retired.  The Bantu returned and either took their salt, or added more gold or took it away. This exchange continued until there was no further exchange.  The transactions took place with neither party seeing the other, it was called ‘silent trading’.

Silent trading took place for hundreds of years, but how did it come about?  How did such an extraordinary level of trust come about between two violently opposed groups?

Given our usual assumptions about trading, an ‘incremental’ solution doesn’t seem possible; they obviously found another way round the problem.