[obedm]

He acknowledged their existence but as far as I know, he, not anyone else, said that slaving people was "wrong" or shouldn't be done.

It's important to note that Ceneca and the famous stoics (Marcus Aurelius for example) were rich and benefitted greatly from the status quo.

[hutzlibu]

So now I think it is a shame, that I barely ever heard of him, despite having heard from all the others great (but slavery endorsing) greek minds. And sure, slavery was very common everywhere at that time. One more reason maybe to celebrate early free thinkers?