So what good is shorting a stock do then? You can't use the money you get from selling it short.
The purpose of shorting a stock is not to get the funds to invest again. It's to take advantage of a possible and potential downfall in the stock's price. Should that short position doesn't pan out the proceeds that you get from shorting the stock is the only thing that you can use to pay back. Think of the proceeds from shorting the stock as the money that you earmarked to buy back the stock in the future to return it back to the broker.