The amount of 50 bitcoins is about a million and a half dollars, and if you want this amount as cash, it will look like this^[1].


I do not think that any sane person would try to do such a deal in a public place, and therefore you will not find anyone willing to do such a transaction with you here.

You have two options:

 - Either you go to OTC markets^[2][3][4], which are highly regulated markets, with prices starting from one million dollars.
 - Wait for government auctions where sized coins are sold.

[1] https://propmoney.info/prop-currency-dimensions.html
[2] https://www.coinbase.com/blog/announcing-new-features-and-services-to-make-trading-easier-for-high-volume-customers-in-asia-and
[3] https://www.kraken.com/features/otc-exchange
[4] https://www.binance.com/en/OTC-Trading