The blockchain is public and transparent. Using exchanges with KYC and ID verification, it won't be difficult to link purchases to individual citizens and hit them with taxes.
There is a law that you can't pay tax twice on the same purchase. It's not like you sold a stock and then bought something with the cash. That's 2 transactions. There will be a lawsuit
purchase of stock (any securities) is not taxed. Profits are. Loss is deductible from profits. Get yourself a CPA. Every honest citizen must pay as little taxes as possible.
Who helps your gov to write the tax law? The Big 4 and CPA lobby groups. Now you know, why it's so complicated. Did few trades cap gain/loss is nothing complicated if you can read