1 Iki Piki: You know, I wouldn't have thought it would be economically viable to ship goods over interstellar distances.
2 Serron: You just need the right sort of goods. Small things that people will pay a huge premium for.
3 {beat}
4 Iki Piki: We're carrying contraband, right?

In many science fiction settings it's simply assumed that there will be some sort of interstellar cargo trade, plying the spacelanes carrying various goods to and fro. The analogy to the Earth's colonial era and the vast trade fleets of ships that criss-crossed our oceans is irresistible and full of story-telling potential and romantic flavour.

The viability of such a cargo trade depends heavily on the scientific assumptions about space travel, however. If space travel is long, tedious, and incredibly expensive, it won't be economically feasible to transport much of anything. Your most lucrative "cargo" in that case is probably passengers, and there simply won't be room to carry much of anything else beyond what's needed to keep them alive to their destination.

There's also the fact that anything you can make on one planet, you can almost certainly make on another planet using the local raw materials for less expense. So there's no real incentive to carry cargo around. Just about the only things that you could possibly make a profit on are "art" materials which may have no great intrinsic value, but are valuable because of historical context or simply because of desirability amongst a rich elite. For example, someone on Epsilon Eridani IV might desire a genuine work by Leonardo do Vinci and be willing to pay for it.

This scenario breaks down once interstellar travel becomes cheap and common. When you can zip over to Tau Ceti II on a whim to visit grandma, then it's probably commercially viable to have ships that carry quantities of locally produced spices, exotic woods from trees that (so far) only grow on Earth, pottery in that distinctive Earth style, and so on.

But at many points along this axis there are niches where illegal goods become worth transporting. You just need goods that are illegal enough that you can command any premium you name for the discretion and risk involved.

EDIT: Readers have kindly pointed out two more-or-less scholarly articles about the economics of interstellar trade:

• One by gaming writer Greg Costikyan (discussing it in gaming terms), and
• One by Economics Nobel Prize laureate Paul Krugman (discussing it with some truly serious economic theory!).

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2022-09-04 Rerun commentary: I saw just a few days ago an interesting fact(oid) on what products the United States imports. Measured by weight, the top three imports to the United States are^[1]:

1. Furniture
2. Bananas
3. Artwork

However, measured by value, the top imports to the USA are^[2]:

1. Cars
2. Computers
3. Medicines

I don't know about you, but this just makes me want to rank everything by its weight rather than its value from now on.

[1] I didn't note any reference at the time, but I managed to find one that confirmed it.

[2] According to the OEC. Different sites give wildly different categories for this, probably because they're splitting or combining things in different ways.