There's a huge difference between market and limit. The best bid or offer can vanish in a flash, leaving you with a fill as bad as $199,999.99 (if you're a buyer) or $.0001 (if you're a seller). You may be able to get it busted, or you may not. Why take this risk? Modern electronic markets were not designed with market orders in mind. The old Island ECN is more or less the basis of all securities markets on the planet, and it did not allow market orders at all -- every order needed a limit. At best, you should input a marketable limit order. If not, you're taking a huge risk just to avoid the "work" of typing a single number.