More and more places I go in other states besides Utah, try to scan IDs when purchasing alcohol.

California grocery stores scan ID too

With digital tokens being generated by a user (the seller) on demand, you could have a bond system where the seller places something costly on the line, that the buyer can choose to destroy or obtain. For instance, if Alice gives her age token to Bob, Bob can (if he is a troll) invalidate the token in a way that requires Alice to go to a physical location to reset her ID.

I imagine this could be done with appropriate zero-knowledge measures so that the combination of Alice's age token and Bob's private key creates a capability to exercise the option, but without the service (e.g. a social media site) knowing that the token belongs to Alice, and without the ID provider (e.g. the state) knowing that Bob was the one who exercised it.

While honest customers have no reason to make use of this option, if Alice blindly sells her tokens to anybody willing to pay, there's bound to be some trolls out there who will do it just for the laughs.

This is far from a perfect system since a dishonest site could also make use of the option. But it theoretically works without revealing anybody's identity (unless the option is used, and then only if the service and the ID provider collude).

'consequences for giving them to kids are severe and the room for meaningful profit is close to zero, same applies here.'

I don't think it applies, the difference is that codes are digital and can be sold over the internet, anonymously, in a scallable manner.

I still like this solution because all the solutions I've seen have flaws and this one being so easy to explain makes it great to campaign for.

You're doing a huge logical jump in your first point. Alcohol and cigarettes are physical goods, digital ID is not, but you're proposing a system that turns it into a physical problem. I'm merely pointing out that's what you're doing and the issues with it.

Second, it doesn't matter what it costs, it's inconvenient and I already spent time (possibly money too) obtaining a government ID... on top of a theoretical mandate that says I need to show the ID on a bunch of websites.

Third, I'm not sure I follow your point on alcohol restrictions creating winners? The non-profit idea could potentially be good, but I'm not hopeful that real world legislation would be crafted that way.

EDIT: also more on #1 and "severe consequences" for re-selling... yes that's exactly what we want to avoid: creating more reasons to put people in prison and a bigger burden on law enforcement and the court system.