If you go to places at high altitude, they literally sell compressed O2 in cans, at oxygen bars, etc.

It is not a requirement per se for you to survive in those environments, but boy does it feel good to get some O2 if you don't have enough red blood cells in your veins to get the oxygen your body needs to feel comfortable.

The same is true, IIRC, in Beijing, where clean air can be hard to come by.

I could not possibly think of a more poorly contrived "counterexample" than that. But perhaps it's sarcasm and the joke flew over my head - in which case have my apologies.

Why is it a poor counter-example? I thought it was a pretty good observation.

I suppose the only reason we don't usually experience air scarcity is because the ecosystem services of earth do a really good job providing it.

It's easy to imagine this not being the case. There is clean air scarcity in polluted cities, and of course in some space environments, air might be considered quite valuable and "hoarded".

Water scarcity and water use rights are totally a thing though.

Exactly. I was giving an example of what post scarcity looks like.

The "air market" is effectively post scarcity for most people (baring the exceptions that you mentioned).

Other markets will become similar to the "air market" when we get really good at producing those things as well.