The nastier issue isn't interstellar trade but interstellar conquest. Trade between stars is very expensive, even for a fairly sophisticated civilization (think of the opportunity costs of waiting decades for a package from interstellar Amazon to arrive!). It's usually much cheaper to make things from local materials and swap data packets with your stellar neighbors.
The problem, of course, is that eventually you might well exhaust the resources of your local solar system. A sufficiently long-lived civilization will do so, no matter how sustainable they are, unless they manage some kind of trick to conquer entropy or basic conservation laws.
Which means you'll need a worked-out interstellar legal framework for laying claim to uninhabited solar systems as destinations for colonies when the time comes to "move house".
And all of this is presuming you're operating at a high enough level of technology and civilization to even gain entry to the Interstellar Club in the first place.
I disagree. Even stars run down, and there's also the issue of drifting debris clouds and various other "Natural Disasters IN SPAAAACE!". Though, yes, admittedly, a star-faring civilization might be developed enough that "moving house" becomes an applicable metaphor: if your old star runs down, just move to a new one that's unoccupied, and since life is rare there will be plenty of those.
Hmmmm... Now I'm imagining a huge set of future Highly Developed Civilizations conducting some kind of scientific conference to share their progress on the matter of the Last Question, since they all realize the bind they're in and would rather work on the problem scientifically now to get increased or unlimited resource sustainability rather than pillage the universe and end up fighting wars with relativistic weaponry in a few billion years.
The problem, of course, is that eventually you might well exhaust the resources of your local solar system. A sufficiently long-lived civilization will do so, no matter how sustainable they are, unless they manage some kind of trick to conquer entropy or basic conservation laws.
Which means you'll need a worked-out interstellar legal framework for laying claim to uninhabited solar systems as destinations for colonies when the time comes to "move house".
And all of this is presuming you're operating at a high enough level of technology and civilization to even gain entry to the Interstellar Club in the first place.