[hm8]

The grocers' price is irrelevant to the discussion here.

The guy with the Ferrari would still outrun you and offer you a new price, with a margin just enough for them to be profitable, yet not substantially large as to talk you out of the deal altogether.

[fleitz]

Yeah, so if you're not a fucking idiot, instead of taking out a front page newspaper ad announcing to the world that you want to buy ALL THE MILK.

You should instead send 4 or 5 trucks to each location and buy the available milk, the guy in the Ferrari cannot outrun you because you're already there, and your order filled before he even got there.

Now imagine that you have at your finger tips a giant constantly updating database to the nearest millisecond about how much milk there is at every grocery store, but you still decide to take out a front page ad and announce your plans in advance, instead of breaking up your order into multiple parts and sending each to its own store. If you did this, everyone would laugh at you like they are at IEX and Capital Group.

[hm8]

Like I said in the original comment, it is pointless to get bogged down in the analogy because it doesn't reflect the truth accurately. However, you guys are continuing down the path an down-voting a perfectly legit argument. The purpose of an analogy is not to maintain fidelity to the original scenario but to simplify it to convey the point of author (otherwise it ends up getting as complicated as the scenario). The analogy is only a coarse approximation of the actual scenario, a best-effort attempt to drive the point across. I'm disappointed with the unreasonable down-votes and the explanation provided.

[ycombobreaker]

On the contrary, the grocers' prices are the very heart of the matter. You see, the Ferrari neighbor may be HFT, but so are the grocers. And every grocer wants to sell their goods at a newly-higher price if they get wind of a big buyer, because it will cost them more to restock if you buy everything. So they will naturally raise their prices. If you buy from the grocers one at a time, expect them to call the other stores (ok it's a chain grocery) to alert them. You will pay a lot for your last milk purchase, unless you stop buying because the asking price has gone too high. If you instead have milk trucks make synchronized purchases across town, you will pay the same price everywhere, and all of the grocers will be upset once they realize what happened. If you do the synchronized milk truck plan enough, the grocers may permanently raise their milk prices at the expense of losing a few other price-conscience buyers.

In reality, this occurs i the stock market because the "national market" consists of something like 15 different exchanges in a big distributed system, so there are race conditions. HFT market makers trade on all of the exchanges, and adjust their prices based on trading demand seen from other exchanges. They spend a lot of money on fast networks between the exchanges, so that they can be/beat the proverbial ferarri. But synchronized trades a la tgemilk trucks or Katsayuma's "Thor" cannot be raced against.

IEX is intended to partially commoditize the Thor approach: take the advantage away from the grocers (HFT, market makers) and give it to the big milk buyers (institutions like hedge funds and pension funds). Retail trading is not affected one way or another.

[harryh]

Once the grocers raise their prices, there's no margin left for the Ferrari owner to capture.

[hm8]

There is still a margin if the intended buyer was willing to pay more or would just want to buy the milk at whatever price offered. Of course, the grocers could keep raising the price arbitrarily but there is still a delta that the guy in the Ferrari could overcharge. I think you understand the point we are trying to get across but just trolling it. I'd have appreciated an honest discussion rather than a trolling attitude.

[harryh]

I'm not trolling.

The situation I'm describing is literally what happens in the real world. If a large trader wants to buy a big block of stock such that he can't fulfill the order on a single exchange he has to be pretty careful about how he executes the trade or the market will move against him. As soon as he purchases all the stock at the market price on a single exchange those selling stock on other exchanges will raise their prices.

It's very important to understand this. The price doesn't (generally) rise on other exchanges because someone swoops in and buys up all the supply. It rises because the people who were selling in the first place change their offers.