Surely the "consumer choice" argument doesn't apply to the used market? Besides, since LA is the city that practically invented smog why don't they have the right to regulate their air quality as they see fit? An emissions standard that works well in the great plains may not work so well in the great cities.

No it doesn’t. An agreement to make new cars with better emissions does not affect the used car market. Now, a regulation about enforcing those emission standards... that would affect the market.

That's not how these regulations work. There are no controls on the resale market in California. There is an emissions inspection regime but it's for SO2, not CO2.