I think there's a tautology hiding here. It's unstated, so I'll give you the benefit of the doubt.

The tautology is that what people exchange for has value, but it has value because people pay for it. Specifically that it has the value people are paying or more.

This is a sort of free market idea: why would someone swap for something that isn't good for them? People must be swapping for stuff they want. To a degree this is totally reasonable. Why does a guy buy a razor if he doesn't think shaving is worth it?

The problem with this kind of thinking is it can never check itself. There's no independent way to give an item a value, so how do you know the exchanged values are correct? There's an assumption implicit that the market gives everything the right value, but it's not checkable without some other way to come up with a value.

And there are reasons to think this value might be wrong. For instance externalities, but also plain old "consumer misjudged the value".

I met a guy who exploited every trick in the book when building his business. He always picked addictive stuff. Online casinos, vapes. He would also put in fine print that people were buying a minimum subscription, so they would they surprised when they got billed a second time. On top of that he found a way to trick Facebook into showing his advertisements that were against the ToS. I doubt anyone got what they thought from this fellow, but he's rich now.

This isn't to say everyone who sets out to just make money makes a mess, far from it. I'm sure there are people whose get rich quick scheme is genuinely useful to other people. But it does mean we need to keep eye on behaviour to make sure there aren't abuses.

> but it's not checkable without some other way to come up with a value

which is the whole point - the value doesn't exist independently of what someone else values it for their own purpose. There's no objective measure of value, like the objective measure of some quantity like weight or energy.

So the free-market assumption is that everyone is always selfish but rational, and thus would only transact if the resulting transaction is beneficial.

in all the cited examples of otherwise, the transaction is fraudulent - someone tricking someone else (which is, technically already illegal, if somewhat un-enforcible all the time).

Anyone who has spent time around addiction and those afflicted by it can tell you it causes more pain than enjoyment. There is a big difference between someome getting high for fun and someone who feels they no longer have a choice but to be high. For what it's worth I am pro deregulation of drugs. I just think it's a good example that money changing hands doesn't imply a net positive had occurred. People pay for guns they use to murder people for another on the nose example.

I don't think anybody objects to the enjoyment of drugs or slot machines. They object to the addictive distortion of the enjoyer's mental state. The free market notion of value surely relies on the rationality of participants. Saying "it is rational for an addict to satisfy their addiction" is... well I suppose it is an argument.