Hey mate congrats if you really discovered that you own such large amount of Bitcoins with you when the prices are surging above $30k at this time but if you want to sell your Bitcoins you need to be very careful and follow some steps for processing such transactions.

"Financial restrictions"
There are some financial restrictions over the exchanges or say limits set up against your account and you can withdraw only upto that much amount so you need to sell your coins on patch basis meaning selling in portions which will in turn not lead to price dump situation in the market because you are saying about 50000 Bitcoins which are more than $1 billion and it can lead to panic in the market with single transaction.

1) Sell over any Exchange:You can sell over any legit exchange as you know if you are not new to the market but be sure they don't freeze your account on temporary or permanent basis as you can loose access to your wallet

2)P2P: You can sell directly to anyone in your reference and who is willing to buy that much of coins from you.It benefit are you can trust that person and moreover you can avoid government regulations and taxes and security breaches over the internet

3) OTC Excganges: You can directly contact with the buyers here without any third-party party intervention and sell to them directly but it's difficult to find buyers of such large amount of coins.

Here is list of some of exchanges and their rates on which you can sell large amounts of your btc
Sell your coins

Its really wonderful feeling when you discover you became rich at very unexpected moments best of luck to you OP.