In StarCluster, generally speaking, there are few things which are worth the cost of interstellar transport. Generally speaking, a system will be able to manufacture anything it needs internally, but there are exceptions:
Works of Art
Artists are not rare, but great works of art may be worth enough to transport from one system to another.
If people on Humbolt have a taste for Hammerklavier wine, then prices will rise high enough to justify importing it.
News is always salable, and takes up little space.
Licenses for designs need transportation. Low mass, high value.
Samples and Prototypes
Samples and prototypes may be sent from system to system for a potential licensee to judge applicability preparatory to signing the license.
People on Ginger in their high tech world are not going to spend hundreds of hours hand-crafting a rug, but the poor people of Hatrack do, and handcrafted rugs are sooo in!
Speaking of what is 'in', Fashion always is!
High Tech Devices
The companies and government of Shasta cannot make the tools that make Locust's robot, but they can pay for the robots themselves, in some cases! remember, each tech level difference is a factor of ten in cost.
Another low-mass, high value cargo! Always, the Data Must Flow!
Pretty obvious, actually.