so in several blocks you spread out your orders to sell 15% of the currency. Well I am no rocket scientist, but I would think that still you would run into some liquidity issues. Actually it might create more of a panic. Imagine a 100,000 BTC sell order, then another, then another, then another, .... That would probably be more panic creating than a single million BTC sell order.
And by selling the coins, your entire attack is based on the false chain you cleverly made so you get one shot to make it pay off.
so this magical instant selling is to me nonviable, which means the N@S will cost you the amount to acquire the stake, so a lot at stake.
I hope James saw the following comments pointing out that shorting is a means to profit from the destruction of a currency as alternative to needing to sell or double-spend:
So it I understand this correctly an attacker could borrow rather than buy say 10% of the target POS coin. This could be done for example using a pirateat40 type scheme. Sell half of the borrowed POS coins short, and use the remaining 5% of the borrowed coins to launch the attack. This would cause the price of the coin to collapse creating massive profits for our short seller / attacker.
Your assumption ignores the possibility of profits from shorting a currency, large bets, or eventual gains from investments in other currencies when the competition is removed.
Simply dumping a large stake on an illiquid market isn't as profitable as repeatedly manipulating the market and taking profits in another currency before taking one large exit with a leveraged short that is assured when one performs a 51% attack.
