Doesn’t matter if their wealth is illiquid, they can still pay a cash tax on it. Us mere mortals, whose major wealth is a house, pay a wealth tax on it every year. (in fact, considering that most homeowners still have a mortgage, they’re paying wealth tax on more than their actual equity) Most billionaire wealth is stocks, bonds, and real estate which are easily valued
What you’re describing, paying taxes when a purchaser divests assets, is exactly what we do now: a capital gains tax