Commerce in Interstellar Civilizations

When you watch (or read) an interstellar space opera have you ever wondered what all those huge cargo ships were actually carrying? What could be so valuable that it would pay to ship it from one star system to another? What could they ship that the destination planet couldn’t make themselves?

In a word the answer is slaves. Slavery is a natural social institution; humans like to have “people” waiting on them. To have slavery, however, you need a permanent outgroup that serves as a source of slaves. The difficulty is that with time outgroups meld into the ingroup. Attempting to maintain an outgroup within a culture is a source of instability. The easy solution is to trade for slaves. Imagine two worlds, alpha and beta, each having a cultural system with winners and losers. Question: What do they do with their losers? Answer: Alpha ships its losers to Beta where they become slaves. In turn Beta ships its losers to Alpha to become slaves there.