I understand why they would want to do that...I'm just not sure if they realize it is at odds with the whole nature and point of shorting to begin with. If a person's transactions have no real effect on the market, it is incredibly easy to fuck with people if you know there are a lot of people leveraged (recent starfish problem, and now an "interest" problem).
Also...in the real world 10,000 leveraged buys is a 10k move in the marketplace. If you do that on bitcoinica, you're exposing yourself to 10k risk without the real world benefit of just buying 10k shares.
We match internally only when we can. If one customer buys 100 and the other sells 50. We just buy 50 from the market, instead of buying 100 and then selling 50, which merely removes liquidity and increases transaction costs.
Like I said, I understand why you do it. What is your window to buffer a matched order?
It's not a time buffer but an amount buffer. We don't disclose the exact amount, but it's definitely under 1000 ฿.