I am sorry that you fail to understand fungibility.
Fungiblity is a legal term of art. It means that you exchange for value, in good faith, and can prove this. The use in common language, that of a good that can be exchanged for another, has no place here, sorry.
The issue is one of law, and when discussing law, you need to use the term of art detailed in law.
Language moves and alters in common parlance. In law — as it is a predicate system of past cases — the same does not apply.
It is not technology, but the legal definition that matters. Zcash cannot ever be money nor fungible, as it cannot be associated with another party. There would need to be an invoice linking it.
Bitcoin IS fungible.
Fungibility comes when money is exchanged as consideration (and without knowledge of crime, etc.) with another for valid consideration.
Here is a nice review of a book I recommend you read: The Legal Aspects of Money, by F.A. Mann.
Trust me, like most in this space, you need to read it, as what you are spouting is utter rubbish. Sorry, but that is being nice. Being popular, having many think this way, is of no value. It is not what the law is, and it is not fact.
Notes ARE perfectly fungible.
As a merchant, you have no obligation to check the history. The fact another can is of no import. Once you have received the note as payment, you have good consideration, and the fact is, under the laws of money and currency, you cannot have it taken.
I suggest that you have a good read of the law of money and currency.
And, this anarchist BS about stopping government… does not work.