You cannot short sell a stock at or below the bid if the last bid tick was a downtick. A downtick is defined by Nasdaq as the condition when the current bid is lower than the previous bid. The simplest way to tell if you can short sell a stock is to get a quote on it (see "market maker quote boxes" later in this guide) and look at the bid tick symbol just to the right of the stock symbol. If it is a down arrow, you cannot short sell the stock at or below the bid. If it is an up arrow, you can short sell the stock at the bid or higher. The red down arrows and green up arrows in your Ticker window also tell you if the last movement was a down tick or up tick.