Various products- Batteries for space ships, rare metals... Commodities that are in high demand by various planets whose sources of said materials are limited. By trading with them it is also possible to secure alliances peacefully, instead of having planets fight over sources of materials through war.
I hope that someday I will be able to trade for the freedom of my home country, and my home planet. I also hope that someday people would realize that it is money, not ideals that is the main cause of war.