For a wealth tax on smallish amounts of wealth, I agree those are problems. But a wealth tax is usually contemplated as being on large fortunes. That removes th

For a wealth tax on smallish amounts of wealth, I agree those are problems. But a wealth tax is usually contemplated as being on large fortunes. That removes the need to worry about the valuations of small stuff. And it also makes encouraging spending rather than accumulation a feature rather than a bug: one purpose of a wealth tax on large fortunes is to prevent large fortunes from accumulating and being maintained indefinitely, i.e. to prevent the emergence of a kind of aristocracy. Thomas Paine proposed an early version of a wealth tax for that reason. His version was something of a hybrid between an inheritance tax and a wealth tax: it was an annual assessment on the current value of inherited estates. That has some conceptual advantages: unlike a straight wealth tax it exempts self-made fortunes for the lifetime of their maker, and unlike a straight inheritance tax it doesn't happen all at once as a single lump sum (which can be problematic for inheritance of hard-to-divide things as well as easier to dodge with instruments like generation-skipping trust funds). But in a modern context it has some administrative disadvantages, because it requires long-term tracking which portion of one's net worth is inherited vs. not.