There are a surprising number of edge cases out there.

Quite a few assets can never clear a market — they have value in some abstract sense but no concrete sense. For example, assets that are legal to own and transfer but illegal to buy or sell.

Some commodity assets have value that it is nonetheless not always transferrable. A common example relevant to wealth taxes is intangible assets where value is bound in who owns it and not the asset per se. Most of the value vanishes the instant you transfer to e.g. the government.

Another common issue is that wealth taxes can directly conflict with existing load-bearing contracts. As a practical matter, these government can’t just void most contracts, including contracts the government is a party to, for the purposes of generating tax revenue.

All of which is why most real-world wealth taxes limit scope to a handful of liquid, legible securities and similar. But as a percentage of wealth, these are pretty small so you don’t collect much revenue.