Probably by those with the "best" record of selling people into slavery - Muslims of Arab ancestry. 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.
There are some Albanian gangs involved, but this really isn't a "muslim problem." The worst offenders are the mafia in the FSU who traffic attractive Eastern European girls who have a huge wage gap with Western women, so they jump at the chance when they're told the creeps can arrange a legitimate job for them.