It's not like the money is just sitting, liquid in a vault like Scrooge McDuck.
Billionaires already routinely sell billions in stock "at once" (meaning, per quarter or similar, not a $1 billion limit order on Robinhood...), so on that one, we can empirically suggest "not much of an effect on the larger economy".
Randomly chosen examples:
https://finance.yahoo.com/news/bill-gates-liquidated-1-7-180...
https://www.reuters.com/business/autos-transportation/elon-m...