Well... I doubt Sugarland is selling out "slaves." They are probably prisoners that will be enslaved by those who pick them up.
Commodities can be tricky like that. Isehara is already buying the "prisoners" commodity, but that is only appropriate for Blood Dragon and Golden C prisoners as it works for short range routes only. When it comes to long range routes, it looks like it just doesn't fit the economy properly... which I guess is where "slaves" came in. Either way, the GMG still calls them "Human Resources."
The fact is, there's a lack of a uniform appropriate commodity for the GMG to used for forced labor. It would be nice to see POWs. (Prisoners of war)