The selling of a security that the seller does not own, or any sale that is completed by the delivery of a security borrowed by the seller. Short sellers assume that they will be able to buy the stock at a lower amount than the price at which they sold short.
Or, in layman's terms:

Selling short is the opposite of going long. That is, short sellers make money if the stock goes down in price.
http://www.investopedia.com/terms/s/shortselling.asp

For example, if I sold short at $5 and then later on bought it back at $3, I would have made $2. Here is a more in-depth explanation: http://investsmart.coe.uga.edu/c001759/guide/trading5.htm .

I doubt that this would be that hard to program in, just take the stock scripts you already have and change a few plus signs to minus signs.

Simple concept, but I would be glad to clarify/talk about it. Thanks!

I was thinking about this too.. if you want to make a stockmarket then selling short is a part of it. why should a person only be rewarded when they can guess which stock will go up.. as well.. when the admins decide to crash the stockmarket then atleast some can benefit from it.

Basically in more Layman's terms.
you "Borrow" a certain amount of stocks from a person (or bank) and sell it. lets say you sell 100 shares of X worth 5 geos each, you make 500 geos. now lets say X drops down to 2 geos per share, you buy the shares back at 200 geos. and give it back to the person( or bank) you borrowed from. you keep the difference which is 300 geos.

Guys, this is a war game, not a financial strategy game ;)

Even in real life selling short is not avaiable to everyone, but considered a bit exotic (at least my bank tells me so :( )