Hello,
From what I read, someone who sells btc sometimes has trouble receiving the fiats in its bank account. If the sum is large, the bank turns on all its red lights. I guess it's not a problem for someone famous, but for an ordinary citizen, how it happens if he lives modestly, recovers his private keys that he lost 10 years ago and finds himself with millions?
If a whale is popular, we've just seen how they've been exposed that they've been selling through huge exchanges as well just like what has happened on the FTX. But if this just an ordinary person, I wouldn't worry about him/her because he's bright enough to buy bitcoin as the earliest stage and selling it would be easy. The regulations will depend on where he is found and I am sure that he won't just sell all of it on an instant with just one exchange. Scattering it and spreading it across various exchanges could be his option and one thing is for sure that he can't skip the KYC part because if he wants to sell and have no problem then that's what he has to go through. Not unless, he knows a huge place where he can OTC or through P2P without having any issues.