Earlier quoted context omitted.
> The theorems about the efficiency of a free market assume perfect information. Huh? That comparatively freer markets make people better off in the real world is an empirical observation, and doesn't rely on theorems. (You can make a few assumptions and prove a few theorems, if you want to. But it's not essential.) > If someone reads Consumer Reports and decides that brand XYZ is making a good product they want to b…
> Btw, that's not an externality. That's just a regular cost. It can be either or both. Information discovery can simply be a cost of transaction, sure, fine. Find out about the stuff you're thinking of buying. Regular transaction cost. Where there are markets and methods of transacting in those markets where one can assume the product that was being sold previously with a given name is precisely similar to the one b…
The 'perfect competition' model is indeed not applicable here. But that's not a problem: we know empirically that markets work (and work better the freer they are). The theory just gives us the intellectual tools to investigate why that is so.
What you are describing is still not a proper externality. In the same sense that eg having a competitor is not described as an externality.
(Or similarly, given your logic, the existence of shops where people have to pay for stuff would be an externality, because now they have to make sure that they are not in a 'shop' where everything is free first. The example is admittedly a bit silly, but if you substitute 'hospital' for 'shop', it's somewhat applicable to the UK.)
About 'perfect competition' about real world competition: you might like to have a look at https://econfaculty.gmu.edu/bcaplan/compet