I don't recommend you to sell at once 50,000 BTC simply because if something goes wrong in the exchange they will either flag your account and it may take a while to get your funds back. Plus, if you do end up paying taxes in your country, you're in for a ride.

You can go follow two roads ( I believe ):

Road A - Slowly selling your BTC in an exchange of your choosing and enjoy your life as it goes. If you don't like exchanges that ask for personnal information (ID's and such) you can try P2P services (localcryptos/Hodl/bisq).

Road B - Exchange big amounts of money and hope that your bank doesn't flag you. This is a more risky move, but provided that you've got proof of purchase or you can explain how you got the BTC I suppose it would be OK in the end. Remember that no one knows what the future may bring and BTC can rise even higher in the future.

Either road that you end up choosing, I'm mostly positive that you can't sell such a high amount of BTC in one single day (or even if you could, it could take a while to transfer such a high amount of BTC to your bank). It would also be interesting to see how it would affect the market.

As always, if you own indeed a vast amount of BTC first step would be to take them to a cold storage and put them inside a nucler bunker in a remote area. Then you can calmly decide what to do next.

I hate it when you just remember that $1.75 billion worth of assets that you left lying around.

OMG! Congrats!!!

Agree with RickDecard: Nuclear bunker + Sell max 10 per day

Congratulations if that's for real. But if you just made an account to discuss it, I don't think that's possible if you plan to dump all of it at once.
You can dump that in several batches but I don't know how long it will take you to dump all of it. And if that's that you "remember" it, do you still have it?

The option one is perfect and i think there is no need to sell all this huge amount directly...the most important think is to stay safe and take all the securuty and it seems our friend did because his bitcoins still safe since 2010
Selling in parts is the best option the bank option still risky

Err no it's not possible. Let's not even mention exchanges probably stopping the transaction, heck, just the fact that banks would probably ask wheretf you got your money, which would lead to a lot of issues (especially if your country has no specialized rules for crypto income) which is going to be a pain. Slowly selling it is one option, but volatility is going to rekt you so you can sell it all at once BUT you gotta talk to exchanges/companies willing to. Might take a lot of time, especially again, you gotta have to talk to your bank about where you got that much money. Might even need a lawyer you can talk to just for the sake of that issue.

On Grayscale, what you buy or sell is shares, not truly bitcoin. You can read more at their FAQ: https://grayscale.co/faq/

I don't know their max cap for intraday sell per user. Others who are more knowledgeable can help you.

It is worth to note that if you are selling 50,000 BTC, you will get a huge huge fiat amount in pocket. Therefore, you need to file taxation document and pay your tax, if you don't want to go in jail.

surely the network will be busy and it will make transactions interrupted and delay occurs. it's better to sell per 1 bitcoin, if it's sold then sell it again and again. don't be 50,000 bitcoins for once. It's a big number after all, will it come true? 

If this is true then you must be one lucky beast for finding those bitcoins. Anyway, money laundering laws are in effect all over the world and having that lots of bitcoins would spell instant meddling from the powers that be who want to get at you or your wealth. Be very careful who you deal with and it might spell bad times to those who tread loosely with all this wealth. If you are the kind of businessman like person who started from little and then became rich because people see that you have a physical means of getting wealth then you will not be mistaken as some thug who stole money. Anyway, enjoy your riches.

11 years you mean because the year now is 2021  not 2022 . you remember that means you arent sure if you have those old btc but why you are asking if how you gonna sell them but why not confirm first that you still have those old coins  . 50,000 bitcoins was alot  , i know btc is cheap back time but those old bitcoiners that i heard on stories dont have that much but they can only have a hundred or a thousand at max but you could be different that them , you might be rich that buys btc or a miner but to answer your concern its better if you can sell partially to an exchange and not on the private individual , selling outside the exchange can be risky .