Trading Posts were essentially general stores set up in the North American frontier. The clientele - mainly explorers and members of local Indigenous nations - would barter for various goods, such as furs, which were shipped back to Europe. The fur trade at the time was extremely lucrative, and one of the reasons that explorers were continually moving into the frontier - they were hoping to find more and better sources of pelts. There was a reason Davy Crockett only ever seemed to have one hat.
Many of the trading posts were privately owned, but they were used to push political agendas as well. For example, after the Revolutionary War ended, the British government used their trading posts to encourage the Indigenous Peoples not to sign treaties with the United States.
The United States government took over the colonial fur trade in the colonies in 1796, establishing something called the "Factory" system. The idea was to regulate prices in order stop exploitation of the Indigenous Peoples by private traders. Unfortunately, under the new system, Indigenous leaders were forced to trade large amount of land to get access to trading posts. Private exploitation had become government exploitation. Lucky that's the only time in history that's ever happened. Since that obviously wasn't any better, the factory system was abolished in 1822.