This is all just a guess. It has to do with legality of items in a system. Just because something is illegal does not mean the planet/station aren't producing them. It just means we can't see it in the trade menu. The AI does not use the trade menu, it trades everything produced. The AI trading is what populates the star map, so you see trade routes we don't have access to.
I've seen Slaves (the illegal kind, not the Imperial kind) for sale at literally every large anarchy station I've landed at, such as Apathamm and Hyperborea. Stations that will actually purchase Slaves through the Commodity board are a bit more... weird. At one point, my "home base" was buying Slaves at a very high unit profit (over 1500 each), I was trucking dozens and dozens of 120-load Slaves to that station for a while.
The weird part is that I'm _pretty_ sure the Slaves didn't appear as sellable on that station's commodity board until I hit Allied with the controlling faction there. Also, the local system authority vessels and the station guns _never_ once shot at me or fined me even though I got scanned every single incoming trip. I'm talking dozens and dozens of illegal cargo loads. At the time, I thought was simply a benefit of being allied with the station's controlling faction. However, the weird part is that one day I logged in and there were no more Slaves sellable at that station despite no other change in my reputation or anything. I'd even get special high-rep "Free the slaves" missions like before, but no more ability to actually sell off the Slave cargo there. No clue what changed or why.