I don't think the average price of UTXO coins is going to be acepted for tax purposes. They usually use the invoices and documents from the exchanges to verify at what price the Bitcoin was bought and sold. So if you bought BTC @ $30000 last year, and more BTC @ $25000 a few months ago, and the price goes up to $27000 or something, then they just look at the dollar value of the coins you bought at those particular times.
And they also look at the dollar value of the coins you're selling, and if it's greater than the amount you bought, that's when capital gains tax kicks in.
But what if you don't use exchanges? I mean, personally I have stopped using exchanges, but I keep buying Bitcoin P2P. Let's assume that I wanted to declare capital gains, how else would I do it? There is no way
