Maybe you could move things down the chain: make it illegal for any entity (with perhaps some exceptions for anonymity compliant polling) to ask for voting proof, with very significant felony punishments (large organizational fines, jailtime for individuals).

Or you can just use a voting system where producing proof is non-trivial, like paper ballots.

You may not want to sell your vote, but others will. And the mere suspicion that other people can sell their votes erodes trust in the system, making it useless for a democracy.

Criminals don't follow laws. Plenty of things are felonies, they still happen, often.