What is the main conflict in Citizen of the Galaxy by Robert A. Heinlein?

As the story opens, a starved, scared young boy is being sold on an auction block on a faraway planet. He is purchased by an old, one-eyed, one-legged man named Baslim. He appears to be a simple beggar, but is later revealed to be an anti-slavery agent (with a long history of heroism) who is working undercover.