A trade this large is likely to crater the currency before it can be filled in the absence of trade volume so large its practically impossible for one stakeholder to act in the manner you describe.
They don't have to actually sell the currency; they can simply transfer it to another wallet they control, use their huge stake to manipulate the blockchain saying they now have original wallet + new wallet, effectively doubling their currency. Then, they can cash out slowly, over time, without cratering the currency.
Basically, this is like a secret money printing (double-spending) machine, although I would like to think that smart people would be able to detect the manipulation in some way or another.
1. Cashout - for example send coins to an exchange, sell them for other crypto, withdraw everything
2. Flood the network with transaction where your coin was transferred to your other wallet instead of exchange
3. There are two conflicting transactions, you want the network to select the one where coins stay in your hands. For PoW that requires 50%+ of CPU power, for PoS it requires to have significant % of all coins.
It is like brute forcing card chargeback - merchant had your money and they are gone and he can't do much about it. It makes exchanges more likely a target of double spending attack than the source. I guess higher deposit/withdraw delays&fees would make PoS attacks unprofitable.
The difference between a system that is provably secure and a system that is theoretically insecure but with no known practical attack is a pretty big difference. In the former case, your proof has to be wrong before you lose, in the latter, someone just has to think of something you didn't.