Before thinking this is a free trade, consider that it costs money to short a stock. In order to short the stock, you need to first borrow it from someone so that you could sell it to someone else. When borrowing a stock, you have to pay an interest rate for the shares which were loaned to you.
Some stocks are cheap to short while others could have high double digit interest rates. The rate depends on the willingness of people to lend and volatility.
