When we buy Bitcoin on an Exchange with KYC, those coins are marked, which is why it is important to do good coin control, always putting notes that identify the origin, so we know which coins are marked. In Nunchuk you can block that coins and avoid sending it in any transaction. Mixing coins is also a good practice. But it is best to buy without KYC, but there is a problem here.

Most of us who buy without KYC do not have FOMO and when there is a drop we go out like lions to buy and accumulate, but then no one sells. So we have to figure out how to find some BTC, that brings us to the exchanges. But we must make use of good security practices in Bitcoin.