Probably by those with the "best" record of selling people into slavery - Muslims of Arab ancestry.
That may well be true, but it's irrelevant to this problem.
My first guess would be Muslims of Albanian ancestry, but there are a number of other possibilities including Russians, Bulgarians, Jamaicans or West Africans who work the sex trade as well.
Either way, I believe the penalties for slave traders from 200 years ago are still very appropriate today --- death -- and it should be enforced.