I’ve never been offered drugs by anyone in my life and I couldn’t tell you if someone standing on a street-corner is someone simply wanting to cross the street when the lights change or an undercover cop… or an actual prostitute. What am I supposed to be looking for?
(Yes, I’m on-the-spectrum and don’t get invited to parties, so I’m not representative of everyone else’s experience, but I am being sincere in my anecdote)
Going downtown around 1am on a Sunday morning should get you the full package, at least in most European cities. City parks, albeit edgy and dangerous, are an almost sure bet.
Dealers usually ask you if all is fine and you need help with anything, prostitutes clues are usually attitude and attire, plus they often work in groups. A good undercover cop will be unrecognisable, for obvious reasons (they have to copy criminal behaviour to be successful).
Providers have a harder time than their customers (longer jail time and higher fines) hence the secrecy which makes it diffocult to spott them.
When in doubt assume it's not a professional, if they are they will make it obvious.
In reality most people aren’t going to (or rather, shouldn’t try to) seek out drug dealers and prostitutes on the street. For the former it’s more likely someone will “know a guy who knows a guy”, or maybe use the dark web. For the latter there are websites and classifieds, you just need to know the right terminology, etc (from what I’ve gathered, I have no personal experience...)
So yeah, it’s underground, but not that underground.
(Analogously, this is why law enforcement professionals and the like have been moving to the term "child sexual abuse material" instead of "child pornography": the problem with it is not that it's pornography, it's that it's sexual abuse.)
It is not only entirely possible to fight the crime of trafficking people and forcing them to do certain acts without criminalizing those acts, it is in fact significantly more effective. Imagine, for instance, if Abraham Lincoln had tried to eliminate slavery by declaring that picking cotton was illegal. Not only would it not have addressed the problem, it would have made the problem worse - slaves are in no position to refuse to do an act because it's illegal, and a fugitive slave would be liable for the crimes they committed as a slave.
Prostitution does not have victims. Forcing someone into prostitution has a victim - the unwilling prostitute. This is a crime committed by the trafficker against the prostitute, not a crime committed by the prostitute against the client (or even vice versa).
Again, take the example I gave of slavery. The analogous argument to yours is to claim that cotton picking is not victimless. Obviously there were victims in the antebellum south. But to say that cotton picking is the crime is to miss the point entirely and to hurt the victims of the actual crime. Cotton picking is victimless, unless you think cotton feels pain. Forcing someone to pick cotton is a crime.
STDs are not a crime*. They are a potential downside of sexual activity of all kinds. We are comfortable with all sorts of activities having potential injurious downsides which are not criminal. You can get a concussion playing football, but that doesn't mean giving someone a concussion in the course of playing football (as opposed to because you specifically set out to attack them, of course) is a crime, and it certainly doesn't mean that playing football itself ought to be criminalized. If someone claimed that the only way to stop football concussions was to criminalize the game, we would find that position laughable or at least dangerously authoritarian.
Robberies and killings are a crime, and they can be prosecuted in their own sake. Robberies and killings also happen in, say, banks; we don't say that this is a reason to make banking illegal.
In fact, it is because banking is legal that police and regulated armed guards can be at banks and stand by as legitimate, victimless banking transactions happen, and that a bank under attack can call for the police and know that they will not get shut down. Going back to the example of the fugitive slave - a slave who knows he's been forced into illegal actions is unlikely to seek his freedom and is quite likely to seek the effective protection of his master, preventing him from accessing the liberty due to him in a just society. Similarly, people participating in the victimless crimes of the sex or drug industry generally have little recourse to the justice system. Robberies and killings (and mistreatment of prostitutes) happens because criminals know that their victims do not have the same ability to pursue justice as the general public does. Fully legalizing these industries is the only plausible first step to solving this problem.
* I should probably say that they should not be a crime. Some jurisdictions have such laws, and they are widely recognized to be both unjust and ineffective, e.g., https://en.wikipedia.org/wiki/Criminal_transmission_of_HIV_i...
Concentration of money will cause unscrupulous people to gather.
Realistically, legalization doesn’t end these issues.
Trafficking underage girls is obviously a crime against another human being, it's not a victimless crime like drugs and should be punished.
If you're killing yourself with unhealthy eating in private, it's on you. Despicable choice for sure, and it sucks if a bad environment drove you down this road - but that's on you: don't make me pay for your healthcare.
We just need someone to step up and collect the data.