I agree with a wealth tax in principle, but it's not without many difficult implementation challenges. For example, someone's entire net worth could be held in art. How do you accurately gauge its value if it hasn't been sold in two decades?

Auction it out. Highest bidder on rent gets to keep it.

This doesn't make sense. You're now saying that the government is going to be in charge of inspecting every citizen's property and auctioning it for them?

I'm as liberal as they come, and that just seems ridiculous.

Yes. Perhaps not the government though, what I have in mind is an institution representing the commons and all individuals right to it. So the income from said auctions could not be used for arbitrary government spending but only for the interest of the commons: The ones agreeing to respect the ownership deal by being non-owners of the asset in question. I imagine most of the revenue would just be payed back as a dividend.

Edit: (I do not think of wealth in the form of art though. Imagine most wealth is tied up in capital assets$

There’s a good moral argument about dividing capital into land and other assets.

But I suspect that from an economical efficiency perspective that division is less interesting than between revenue and assets.

It's really more about dividing capital into the kind that is "natural" and limited - land, mineral rights, EM spectrum; some would also say copyright and patents - and the kind that is produced by human labor. It's a bit fuzzy in practice.