Good question!
So in AGOT, they only speak in general terms of Jorah “selling some poachers to a Tyroshi slaver” or “a few lice-ridden poachers,” although the people who talk about this are either slavers-in-all-but-name (Illyrio) or Jorah himself, who has every reason to minimize his crimes.
However, Jorah shows a surprisingly deep knowledge of the slave economy, much deeper than you’d expect to see from a Westerosi lord with only one encounter with this foreign economic system:
“I’ve told the khal he ought to make for Meereen,” Ser Jorah said. “They’ll pay a better price than he’d get from a slaving caravan. Illyrio writes that they had a plague last year, so the brothels are paying double for healthy young girls, and triple for boys under ten. If enough children survive the journey, the gold will buy us all the ships we need, and hire men to sail them.”
My thinking is that either Jorah sold slaves (more than once) in Westeros and/or he did it in Essos (or was otherwise employed in the industry as a guard or an overseer or something). I think the former case is more likely, because it seems somewhat unlikely that he got caught on his first outing.
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