Avitar, Inc. (AVTI)

[k_trader]

Market Makers are permitted to engage in naked shorting.

U.S. exchanges employ several competing official market makers (MM) in a security. These market makers are required to maintain two-sided markets during exchange hours and are obligated to buy and sell at their displayed bids and offers. They typically do not receive the trading advantages a specialist does, but they do get some, such as the ability to naked short a stock, i.e., selling it without borrowing it. In most situations, only official market makers are permitted to engage in naked shorting. Recent changes to the rules have explicitly banned naked shorting by options market makers.

A market maker makes money by buying stock at a lower price than the price at which they sell it, or selling the stock at a higher price than they buy it back. Ordinarily they can make money in both rising or falling markets, by taking advantage of the difference between "bid" and "offer" prices.

Stock market makers also receive liquidity rebates from electronic communication networks for each share that is sold to or purchased from each posted bid or offer. Conversely, a trader who takes liquidity from a bid or offer posted on an ECN is charged a fee for removing that liquidity.

Here is some reference material, please read it.
http://en.wikipedia.org/wiki/Market_maker