In point of fact, mark to market taxation already does exist for various individuals and certainly for large numbers of businesses. Your long comment suggests that you don’t know what that is, and if you’re interested you could read up on it.
The short story is that depending on the situation, a person or a business might pay taxes each year on the value of their assets, assuming said assets had been purchased on January 1st and sold on December 31st, even though in reality nothing was bought or sold. This system is already in place in various ways. It exists. There’s no theoretical problem with expanding it.
There are already large numbers of treaties in place for countries to cooperate in tracking down tax evasion. It might not be as expensive as you think. If you think of it from a practical standpoint, we have lists of the richest people in the world. That’s an excellent starting point, isn’t it?
If some billionaire is claiming that they actually aren’t a billionaire, and that the lists are wrong, when some government is trying to tax them at an exorbitant rate, it’s likely that they will give all of their banking details to said government to prove it. Or they will hide those banking details, and they’ll be forced to pay the taxes.