Because of Disease and war. Mostly disease like smallpox though.
The sino-soviet dispute of the late 1950s was the most important development.
It was A. Bartering. Coins weren't existent that far back, international trade didn't exist because no one knew about the continents to the west, and it definitly wasn't free trade. Nothing was ever free.
The Indus Valley civilians used granaries for c. storing food
Iron, ivory, gold, and salt were common goods
