Once every about 5 trips between two systems i can get a mission to deliver 2 slaves, all you seem to have to do is buy the slaves in the same starpoint go back to the missions and complete the mission for an easy 20 to 30,000 worth of credits.
I've seen this at ports with other things, like Nonlethal Weapons.
I just figure, particularly since it's the Empire, that the mission comes from a nobleman who is above such pedestrian things as shopping in the commodity market.
I used to enjoy this 'bug' a lot, but recently the demand for slaves has crashed.