The problem is that not all people have good intentions. That's why we actually need the FDA, or something like it.

There are a lot of tools to address the problem that some people have bad intentions.

We publish common sense laws, and we have police officers and prosecutors, and then we have a court system to hold people accountable for breaking the law. That's one pretty major method that has little to do with the need for an institution like FDA.

I don't know if a system that relied entirely on tort and negligence and contract law to protect people from being sold snake oil would function better or worse than FDA, but I do know something like FDA (where a bunch of smart people advise very specifically on which drugs are ok to take and which are not) isn't the only option we have.