Hm, from the viewpoint of ingame laws we can explain inability to enslave anyone by the fact that a girl either should commit a very serious crime to be enslaved by the government itself, or be a daughter of slave (or maybe not, it could be a cheap way to get many slaves when/if we'll add pregnancy).
Thus, at slavemerket you can buy and sell traned, expensive slaves, and in prison you can buy cheap, untrained, dangerous ones, as well as sell rm girls for example.
I think we'll need a prison location eventually anyway, might as well add it with ST.