Tip: place limit orders to buy two thirds between the bid and the ask. ie bid/ask of 1.00/1.30 you place the limit buy order at 1.20. If you are selling the option then place the sell order at 1.10. That way you get a good chance of getting filled at a price close to fair.If it is an order you need to be sure is filled to protect your financial health, use a market order. Otherwise, use a limit order.
The first three answers correctly identified some good reasons that a limit order is preferable, but if you write naked options (including those in ratio spreads) you may find yourself in a situation where you need to buy some options quickly to control your risk. When you use a limit order there is a chance the order will not be filled, but if you use a market order you can be sure the order will fill.The short answer is limit orders.
The long answer is that using options successfully (which may involve losing money on options -- therefore "successfully," not "profitably") first and foremost involves calculating and sticking to a strict cost model. In other words, doing the math should give you an estimate of options trading at a discount to historic volatility or , at least, not trading at a premium. Buying at a premium regularly will always lead to broken hearts and crushed dreams.
