I honestly didn't realize people still paid such a premium for aged bitcoins. I mean, before yes, before the 2017 rally I kinda understood it, Mt Gox coins and hacked coins and illegal coins etc but now Bitcoin fungibility is the thing, and there is no such thing as 1 bitcoin is more/less worth than another.

Or can someone correct me?

I think it is probably more likely that the bitcoin being exchanged can be traced back to a theft or hack. There is no reason to need to prove you held coin for 4 years to pay long term capital gains tax rates. You also do not need to provide that level of documentation when filing your taxes, at least in the US, and assuming you are not being audited (audit rates are very low, and I have no idea what level of documentation would be needed in the event of an audit).

I think if anyone were to deal with the OP, they will end up having many merchants and exchanges be unwilling to accept their coin.

Let me guess: taxes? Claiming capital gains while in reality you got the coins in other ways?
As attractive as this offer sounds, I wouldn't dare risk money laundering charges because of this.

The problem with that is that the "seller" can also still sign the same message, and if he too uses it for taxes, it might come back to haunt you later.

This makes sense. In the 1 => 1 transaction you could include an OP_RETURN Output with a message stating this. An example:

https://blockchair.com/bitcoin/transaction/1de99d383743158acfc2d2a21231c226df23c24c1ba4aa1059addcf5e1b348b5