A little more info on shorting (I did the bold, itallics & underline Seems to have to be Margin?):

Shorting is a trading technique that allows you to profit from a stock's decline. A short seller borrows shares he does not own from his broker, sells them, keeps the cash, buys the shares back later at a lower price and returns them to the broker. The profit he makes is the difference between what he originally received for the shares and the price he paid to buy them back. The success of the operation depends on the stock declining in price and whether or not it can be shorted.

1. Margin Account

   • A short seller must have a margin account; she cannot short with a cash account. A margin account automatically allows shorting, provided the trader has sufficient margin or equity in the account.

   Sufficient Funds or Equity

   • The margin/equity requirement for shorting is the same as for buying stocks. For example, if a trader wants to short 500 shares of a $20 stock, he must have at least $5,000 in cash or equity in the account, or half the total amount that is being purchased.

   Share Availability and Margin Requirements

   • A trader can only short shares if they are available in sufficient quantity for shorting. Shares held in a broker's customer margin accounts can be borrowed and shorted by other traders. Every broker has a margin department that tabulates the number of shares available for shorting each trading day. The margin department also decides which stocks can be shorted and the initial margin requirement. For example, some low-priced or thinly-traded stocks, or shares in an initial public offering, may not be eligible for shorting. Additionally, some high-flying volatile stocks may have higher initial margin requirements.

   Automatic Trade Cancellation

   • Every broker's margin policies are different and constantly change, so there is no point in trying to understand the specific shorting rules of your broker. All a trader needs to do if she wants to short a stock is to enter the number of shares and click on the "Sell Short" button. If she has sufficient equity in the account and the shares are available for shorting, the trade will go through; if not, it will be automatically canceled. The number of shares of a stock in a broker's customer margin accounts can vary from day to day and even hour to hour, so a short trade that is canceled sometimes may go through hours or even days later if enough shares become available for shorting.