It sounds like you think orders are executed atomically; that if you put in a sell order for 10,000 shares, 10,000 shares are sold all at once. That's of course not how it works. Even if you don't split your order up into smaller ones deliberately, it is still going to be filled piecemeal by lots of small buy orders. And that's not going to happen all at once either.
If you have 10,000 shares that you want to sell, and you want to sell them all at a specific price, there is a simple mechanism to do that: enter a limit sell order.
Re:Why Is Spoofing Bad?
by Dylan16807 on Thursday April 23, 2015 @10:01AM
(#9426686)
I'm aware it's not atomic. There wouldn't be time to react if it was atomic.
They break their orders into smaller pieces, and build (or rent from their brokers) algorithms to make their big orders harder to spot and less likely to move markets. John Arnold "spent millions of dollars developing a proprietary order-entry system to disguise and conceal strategies from external algorithms."
The "ideal" is that this would not be necessary. That this would provide no advantage, because markets would never try to spot "large intentions". Party B would not temporarily set a price that is less profitable for selling small numbers and more profitable for selling large numbers.
Markets don't need immediate higher-order feedback to be efficient.
And again, don't ask me how you would stop anyone.