When you go to enter a "Sell Order" enter the lowest price your willing to accept for the quantity of coins! The order will be placed into the order book as an offer:

For example, say you want to sell 10 coins, and the last quote is at 180, but the lag is some crazy high number (anything over a few seconds is a JOKE by professional standards but that's another thread), now you know you don't want to sell for less than $190 each, so enter, the quantity, 10, and erase whatever the quote is and put $190. Your order will be placed as "pending" until it enters the book, at which point your coins will be offered at $190 each. If someone is bidding higher than $190, you'll probably sell them to them at $190, if not, your order will remain until you either cancel, and replace it, or someone bids high enough for them!

Simple enough!