The barter and the gold were the most common things used in the Ancient world for trade. The barter, or bartering, is essentially the exchange of goods, or rather one good for another good. That of course goes in accordance to the valuation of the goods, so for example, one goat would be traded for ten rabbits. The gold, on the other hand, was a materialistic thing used for exchange of goods. The gold was very highly valued, so each amount of gold had certain valuation, and according to its valuation the people were able to purchase certain goods with it. The gold was in a many ways what the paper money are in the present day.
The climate in Saudi Arabia, Iraq, and Iran is mainly Desert.
Those places are located around the Middle East area and the Middle East is covered in dry hot sand so it'll make sense that Saudi Arabia, Iraq and Iran is a desert.
