Answer:
The kings of Ghana used the law of supply and demand to their advantage by limiting the amount of gold available for trade, increasing its value.
Step-by-step explanation:
According to the law of supply and demand, when there is a high demand of a product but the amount of the good available for trade is scarce - either naturally or forced - the price of the goods will increase. This is what the kings of Ghana did, becoming a strong force in trans-Saharan trade.