As far as I know, it would be impossible for government to put taxes on decentralized market.
You really believe NFTs are decentralized? This is like saying Tornado Cash is decentralized until the Government made its move. Keep in mind that anything that generates profit is generally taxable in all places unless there's a specific law that says it's not.
I would guess that you could make it decentralized until figured out. No matter how hard you try, if I open a new metamask today, and put all my NFT in there, nobody in the world would be able to track that down to me, that's impossible, you can use something like The onion router as well to make sure that it can't be tracked back to you and you are done.
Basically it is as decentralized as you want it to be, and then when you want to cash that out, there are mixers if you want to use it, but then you could always just pay the cash if you profited and not really care about the tax in that situation, after all making x100 means you can pay the tax.