General:
* Trading center
* Trading station
* Outpost
* Trading depot
* Market
* Barter point
* Exchange point
More specific:
* Frontier post (if in a remote area)
* Fort (if it was historically fortified)
* Trading fort (if it was historically fortified and primarily for trade)
* Factory (if it was a European trading post in the 17th-19th centuries)
* Trading house (if it's a more established business)
The best synonym will depend on the specific situation and the type of trading post you're referring to.