[Suicide Junkie]

I did suggest how to do this early on...

In each system, place a trade hub world or station, and have that place give out jobs to deliver the particular cargo that the system wants.
Starbase Terra could give out jobs to deliver rare ores and luxury items, while the New Texas Starbase would have more demand for items such as Terraforming units and complex manufactured goods.

The only place to get the rare ores that Earth wants would be systems like New Texas.
The only place to get the manufactured goods would be developed worlds like Earth.

There's no real time constraints, since you collect the goods of your own volition, and then visit the Hub World for a 20-day delivery mission to a local planet for a small profit margin.