Interesting concept, but trade balancing and the fact that various human cargo takes up different amounts of space could complicate things.
I would like to interject, however, that slavers can currently do so more than simply pirate other ships for human cargo. While human cargo is their forte, they can smuggle any other illegal good as well, and with larger ships than an independent smuggler or a non-guard id'd pirate. The tradeoff is severely restricted docking rights and the inability to pirate for non-human cargo.
