Happy to indulge. No, it wouldn't be possible to prove it was money changing hands (and in this example, it wasn't!).

Let's just put politics & ethics aside for the moment and talk about practical matters.

There's a question of who has the burdon of proof. Everyone can see that it appears the owner of address A bought something (possibly fiat currency) with their coin. If the taxing authorities know who owns address A, they could send the owner a tax invoice, or a letter accusing they of failing to report a transaction and pay tax on it (e.g. capital gains). They could even calculate the tax owed based on the price when it arrived in the wallet vs when it left.

As a response, the owner could just say that's incorrect, they just moved it from one of their accounts to another. Whether this explanation is accepted or not is up to the taxing authority. If the tax enforcers were savvy, they could say "sign this message proving you control address B". If they refuse, then perhaps the courts would sort it out.

If the tax man doesn't know the payer, nor the payee, then it's a dead end. If they know the recipient, then they could come after them for not reporting income and it pretty much plays out the same as above.