Well, an exchange at exchange is way less time-consuming and provide a counterparty whenever you need with usually a lower fee

If someone want to use btc officially (e.g. buy a flat) he must tax it first. But with the p2p trade, you don’t have the buy price, or at least the proof that you bought it, so you have to pay tax from the whole amount and not only from price difference. Or I’m wrong here?