Trading posts are neutral western camps that will offer to trade with you.
Just go to the camp and speak (press F) to one of the men there and you will be able to see what they have.
They can have a variety of things: from ingredients or mats to HQ weapons.
There will be more than one trading post on the map. Each trading posts will have different items but every person in the trading post will have the same to offer.
You can use any items/materials you want to trade; weapons/bows, arrows/ammo or even just plain wood or stone!
Watch out because if these men get killed they won't respawn and you lose a trading post.
Hint: use items/materials which are gatherable or craftable such as bows, arrows, stone, wood,...