There are different scenarios that can cause grief in either method. Thankfully for you the grief is less in the selling scenario than buying.
In short periods of time, especially when markets open in the morning, there can be bizarre market fluctuations. If possible, whether limit or market, make sure it doesn't execute in the morning. If you can wait for the sale and want a guaranteed price, a limit order makes sure you don't lose a few percentage points if a random drop occurs. If you don't care, you just want the stuff sold and need to use the money immediately, a market order should be fine.
Your broker is legally required to get the best price available regardless the order type market. So these oddities don't usually matter much. If you are finding this choice difficult: flip a coin. You are likely to be right over 50% of the time.