[gd1]

That's bizarre. So let's say AAPL is trading at $120, and you put in a limit order to buy it at $110, then you should never be allowed to cancel it? How long does the order sit there for? What if AAPL's price goes up to $400 over the next few months, do you have to leave your limit order there forever?

[sjtrny]

I don't think fenollop means not withdraw it at all. He means after someone has said "I'll take that offer". At which point you can't withdraw it. So these people will make a loss of enough people take up their offers. This risk is mentioned in the article. It doesn't explain why "spoofing" is illegal though. It should be legal. They are trying to shift the market (so is everyone) through an action that exposes themselves to massive loses.

[kasey_junk]

This system currently does not allow you to withdraw an offer once it has been matched. That is not what happened here.

The reason "spoofing" is illegal is because it gives a huge information advantage to one party that the other parties do not have. Similarly to insider trading. I have no opinion on whether those should be illegal, but if the basic premise of the market changes from "we assume everyone here wants to trade" to "this information is garbage", it will definitely have a negative impact on the price of the traded instruments.

[sjtrny]

The information isn't garbage though because people can purchase the shares that are being spoofed. Even if the spoofer doesn't want it to happen you can still do it. You agreed with me, you can't withdraw the offers once matched so the offers are legitimate.

[kasey_junk]

I think there is some information loss (I'm not sure how much). In a non-spoofing allowed world (where everyone is playing fairly) volume at the back of an order book has implications on the price at the front of it. Figuring out what the implication "really" is, is a differentiating feature that market makers can compete on.

In a spoofing allowed word, that volume is meaningless. Market makers can only differentiate on how quickly they can pull their fake orders and how much risk they are allowed to play with. Not something I'm sure we want to be optimize the rules for.

That said, given the difficulty in enforcement of this particular rule, it may be worthwhile to just throw our arms up and say, "we can't enforce it, so lets make it legal and everyone moves forward on an even playing field".