In message 20010731202223.B32497@welkin.apana.org.au, Kathryn Andersen kat@foobox.net writes
The thing is, I don't think we could say that privately owned slaves were illegal, or that the buyers were from non-Federation worlds where slavery was illegal -- because what were they buying, then? It would be like someone turning up in Australia with a slave; they'd be guilty of kidnapping and wouldn't be able to keep the slave at all!
One of the things I'm thinking of here is in fact the use of convicts as slave labour during white settlement of Australia. IIRC, a lot of the convicts were sold to free settlers as indentured labour, but remained technically the property of the state - and if you turned up with a (white) slave you'd bought elsewhere, you'd be on very dodgy ground indeed.