If it were made legal, I doubt if there would be very much of a market left for "human trafficking".
I'm a Dutch guy and here in the Netherlands prostitution is legal ...
But, that does not mean that all women in prostitution are in the game out of free will. Lot's of girls from the former Eastern bloc are working as a prostitute to get their passport back from the people that brought them to the Netherlands.
Another big problem are what became known as a so called loverboy in the Netherlands and Belgium. Those loverboys are boys who seduce innocent and very often insecure girls into a relation by spoiling the girls with gifts. Later on they force the girls into prostitution, using extreme violence if 'needed'.
Ask google for 'dutch loverboy' to get more info ...
I still I think prostitution should be legal, but thinking legalization will solve all problems in prostitution is just an illusion ....