Who is enforcing this and how?

Liability would be decided by the courts or another form of binding arbitration. Obviously. Harming someone through action or negligence is a tort, and torts are addressed by the judicial branch. Both sides would present their arguments, including any scientific evidence in their favor—the FDA or similar organizations could weigh in here as expert witnesses, if they have something to offer—and the court will decide whether the vendor acted reasonably or has liability toward the defendant.

If you knowingly sell me a car with an engine about to fail, you are in no way accountable.

If you knew that the engine was about to fail and didn't disclose that fact, or specifically indicate that the vehicle was being sold "as-is" with no guarantees, then you certainly should be accountable for that. Your contract with the buyer was based on the premise that they were getting a vehicle in a certain condition. An unknown fault would be one thing, but if you knew about the issue and the buyer did not then there was no "meeting of the minds", which means that the contract is void and you are a thief for taking their payment under false pretenses.

Anyway, you continue to miss the point. I'm not saying that everyone should become an expert in every domain. I'm saying that people should be able to choose their own experts (reputation sources) rather than have one particular organization like the FDA (instance/community moderators) pre-filtering the options for everyone. I wasn't even the one who brought up the FDA—this thread was originally about online content moderation. If you insist on continuing the thread please try to limit yourself to relevant points.