ravron

How about the Moties from The Mote in God's Eye? Their "engineer" sub-species has two dexterous right arms and one strong left arm, and employs the tiny, semi-sentient "watchmaker" sub-species as disposable tools.

Check out Niven's "The Fourth Profession;" it handles this issue by saying that interstellar trade is a one-time event, essentially. Without spoiling it too badly, an alien race skips around the galaxy at relativistic speeds, stopping at inhabited worlds to trade, and then leaving with no intention of returning essentially ever. This frame of mind leads to some interesting revelations near the end of that story.