I have friend who is a whale, once saw him selling about €1,500,000 on Kraken but it was annoying as fuck. I didn't see the whole process but I remember him coming back and selling in smaller amounts, he also called his bank in advance saying he'd be depositing large amounts of money in his account.

Not sure if it's the best way but it's the largest sell I've seen, also this was back in 2017.

It's not that hard to do on exchanges with high liquidity like Coinbase, Binance, Bitstamp, and safer than meeting with someone and waiting for them to wire money to your account while you both wait for a confirmation on the blockchain.

What large sums are you talking about? 500 thousand? 1 million dollars? 2 million? It you're talking about 100 BTC or less don't overthink it. Exchanges are used to this and you'll just dump it into a wall. If you have more than 1000 you're best trading OTC or splitting it into small parts and selling on more than 1 exchange.