after 12 years i remember i bought bitcoin in 2010
is it possible to sell more than 50,000 bitcoins at once or is it better to sell direct to a private individual like grayscale or microstrategy?
Wtf!

That’s over a billion dollars if you’ really mean this!!!

8 You shouldn’t sell that amount of money at once, and I wouldn’t even advise you to sell it by using a centralized exchanges, if I was the one I will make use of P2P exchanges, there are lot of them where I live and they offer good services.
If you try selling it at once, and move it all to a centralized exchange, you might encounter a problem with them as they may try to verify the source or something, and then if you send the huge amount of money to your bank at once you will have a serious question to answer, and it gets worse if you can’t prove it. Selling it bit by bit and using P2P would be best, don’t be in a hurry.