If so, he can't sell it without tanking the value so hard he'd never be able to get a decent fraction of its "worth".
That's not even talking about alternate ways to transfer his wealth out of ETH (eg. approaching his billionaire friend: "how about you sell me that mansion for 100k ETH?")
And again off exchange block/bulk sales tend to go for lower than market price. They are rarely, if ever, executed at market price. So most probably he will have to offer 100k ETH, 4.8mil, for a ~4 mil mansion.
"4.8mil, for a ~4 mil mansion" I agree and that's totally reasonable. Only a 20% hit. Pxtl was saying something much more dramatic ("he'd never be able to get a decent fraction of its worth").
No, the blockchain is the data structure that stores historical transactions.
He could be a large owner of the circulating currency, and he would face issues trying to sell a huge chunk of value all at once. The crypto markets are not very liquid compared with mature financial markets.