Trafficking, and illegal prostitution, will occur even when prostitution is legal.

The difference in legal prostitution is that it will offer protection from the state and will reduce, but not stop, sex trafficking. There is increasing EU-wide cooperation between police forces on this subject, but it is a fairly new type of crime fighting. But at least efforts and time can be increasingly concentrated on this subject when prostitution is legalised, instead of wasting time on the ideological pursuit of eradicating sex workers altogether...