i typically trade relatively large baskets of stocks-50-100 positions..every single day i see how "they" withdraw their bids when i trying to unload(for example long positions) all at once at the current bid price.whatever you say guys, from my perspective it's not fair ,when someone can withdraw their bid OR have ability to place an order before your order can reach an exchange. it's basically front running. if very small tax can stop this abuse and can stop the useless flashers-i would appreciate it and don't mid to pay some extra to get rid of them. in recent months it 's became nearly impossible for retail guy to get into winning trade on 50-500K volume stocks. i did very extensive research and i'm keeping records of my "fill ratio"..it's falling every single day. "adding liquidity" is a bs because as i said above-they do have ability to withdraw their orders.they not really adding anything. you can see 20K shares at the bid on some 100K average stock and you sell 100 at this price or place a sell order at 1 cent above to narrow the spread from let say 2 cents to one and they gone with this 20K order in milliseconds. it's all bs most of the time. at least on stocks that i'm dealing with.