But it's also quite serious, because the specifics of sub-light-speed interstellar travel do pose some rather unfamiliar and intriguing conundrums: Krugman points out that transit times for goods are of course extremely large, and the passage of time itself is a function of inertial frames and the acceleration of frames.
Relativity says that you can keep your roses fresh for Alpha Centauri, but your first customers may dead by the time you get there.
You can teleport a copy to your automated reading device by clicking here.
It's a lot of fun, with a satirical slant lurking between the lines.
The fact being that almost any cargo along these lines (made of the elements produced across the universe by stellar nucleosynthesis and supernovae) is going to be a) most likely available in any system already, b) definitely available for the taking from billions of unoccupied regions of space. I suspect these would fall into two broad categories. A few billion years of natural selection and evolution on any given planet will produce an array of wonderful and useful lifeforms, as well as biologically formed structures (I'm thinking seashells and honeycombs here), that could be unique enough to be of interest elsewhere.
Paul Krugman Research Papers Thesis Custom Footer
No matter how clever a species is at genetic manipulation or creation it will be hard to muster enough imagination to match what springs out of nature's vast playground.
He also shows that, in terms of capital, interstellar arbitrage and competition will tend to equalize interest rates between two locations (planets, ring-worlds, Dyson spheres, whatever) as long as they're in the same inertial frame.
This, as far as I can see, is astrophysically tenable.
But pure art could work too; maybe some lovely Picasso's for GL581, or perhaps a couple of Caravaggio's?
In fact here might be a hint of the biggest market, those items that can be transmitted rather than transported.