Trading Posts act a bit like a market, but are private businesses. Assigning yourself or a worker to 'Sell Goods' at a trading post will allow you to sell any goods in the inventory directly to customers.
Trading Posts are usually used as a supplementary business to sell your goods directly to customers. However, they can also be used as a solo money maker; by watching the markets and buying overstocked goods, storing them until they're needed, and then selling them directly to people.
Get an extra 10% from each sale made directly from your Trading Post.
An extra cart for your Trading Post.
Once you have horse carts the horses will be taught of far away places and be able to haul caravans across country.