damn that's a lot of Funds , 50,000 Bitcoin ? So you're one of the early adopter and one of the person that becomes Billionaire .
50,000 bitcoin is a lot of coins and anyone holding that kind of coins should be a miner or someone that purchased the coins when the valuation was minimum but the question here is that whether he is really having that kind of coins and whether he was sleeping when the price of bitcoin touched its all time valuation last time, it was a global news and anyone invested would remember them.
@OP there are legit OTC traders available if you are holding huge number of coins. You can contact Kraken and they will help you in finding good buyers.