Once it was only the odd ship that stole people from the Land and sold them ot lands over the seas which were supposed not to exist. Then a ship with a flag showing a flaming salamander began to attack any vessel known to trade in slaves, sinking it. One crew member would be kept alive, tortured, then cast ashore to tell the story of a ship with Beforetime weapons that spat fire. In no time the trade virtually ceased, for no one wanted to make themselves a target for such a nemesis.
Sian
What if Salamander began as Seresh, who covered her face to hide her scars and would have wanted to end the slave trade, but then Ariel decided that she was in the way of his operations so he kidnapped or assassinated her and put one of his cronies in her place. Because her face was always covered, no one outside the ship's crew would know the difference. He would have had to replace the crew as well, but I'm sure he'd know enough despicable souls for that.
Ashalinde
did IC mention in an interview once that we had already met Salamander?
Sian
Ashalinde, with regards to why someone would take Seresh as a slave, in The Sending:
We're told that some of the slavers are particularly interested in slaves with special talents such as navigating. In TSK, Jakoby navigates the ship when the coast is out of site because she's the most used to it and this is where we learn of her and Seresh's sailing tendencies of children. Seresh might have made a good slave in spite of her deformities because of skills that she has.