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You do, it's factoring the fact that something that cannot be sold legally can still has black market value, which is definitely the case for stolen art and antiques, protected species etc...

So new parents should be deemed to have received whatever a baby’s worth on the black market?

True, but then you're taxing someone for a crime they haven't commited yet and might never commit.

and stock!

Value is a very nebulous concept. But laws consistently affect the prices of everything from food to real estate to medical care to weapons to labour.

The changes aren't usually zero -> something, but that's a footnote to the general principle.

Prices are real. Value is imaginary until something changes hands.

I disagree. Markets react to election results.