So i got the Ideology and Royalty expansion, playing an ideology that thinks slavery is honorable.
It says that they will get a +10 opinion for selling slaves.
However when i sell the slaves to the empire for honor (to the tribute collector), it counts them as both prisoners and slaves it seems.
So the colonist who sells them gets +10 opinion for selling slaves and -15 opinion for selling prisoners to slavery.
I haven't found anyone else to sell slaves to yet, so i don't know if this is standard everytime you sell slaves.
Moderator Note:
This does not appear to be happening anymore on 1.3.3087. Thanks for the report!