I appreciate your engagement in this discussion. I think the notion of a competitive market as you define it [0] is inherently flawed. To quote:

> all participants are price takers, i.e., passive agents incapable of changing the market price

That makes no sense. If all participants cannot influence the price, then where does the price come from?

Logically speaking, once you abstract away from all sorts of fluff, everything that happens in a market is ultimately the result of the action of one or more of its participants. That is, if participants were unable to change the market price, then why would the market price ever change? Why would there be a price in the first place? The fact is that participants are necessarily able to change the market price, whether you want that or not. The only question is how this power is distributed and what the consequences are.

Having competitive markets in the sense of low barriers to entry etc. is clearly a good thing. But in the sense you've defined them, they are either useless or self-contradictory.

Interestingly, for every market, somebody has the (or at least some) power to set prices. Despite of what you write, most transactions are mutually beneficial anyway. Why is that?

I believe this is actually because the vast majority of markets aren't efficient in the efficient market hypothesis sense, and transactions aren't happening at the margin. For obvious transaction overhead and opportunity cost reasons, most transactions are only ever realized if they increase the total welfare not just by some epsilon, but by a significant percentage.

The really interesting question is how this large benefit is distributed between the transacting parties, and I would wager that the power to set prices does actually have a big influence there. That is, whoever actually gets to set the sticker price (for example) is bound to capture a larger fraction of the benefits than the other party, even if the transaction is overall still beneficial to both.

For this reason, I think the problem isn't even so much that transactions leave people worse off. That only happens comparatively rarely. The much more significant problem is when unequal power relations are exploited to give somebody a benefit that is deemed to be unjustified. [1]

The prototypical example would be the millionaire in the desert who is near death by thirst. If she signs over her entire wealth in exchange for a bottle of water and a ride out of the desert, then clearly, this is a transaction that is beneficial to both parties. Yet only the most psychopathic internet-libertarians would ever think that such a transaction was okay in any sense. The problem was not a lack of mutual benefit, but an exploitation of unequal power relations.

[0] And I suppose others define it in the same way, perhaps even a large group of economists.

[1] By the way, this is the root of the classical criticism of capitalism: that the surplus value of labor is siphoned off by capitalists, leaving workers with only a small amount. Even though employment contracts are typically mutually beneficial in the narrow, technical sense, there is the fact that the employer often exploits the reality of unequal power relationships.