The best sports betting market is Match Outcome because it is more common and easy to research on. However, you can also consider Outrights in horse racing since they have competitive odds.
To summarize, short selling is the act of betting against a stock by selling borrowed shares and then repurchasing them at a lower cost and returning them later. It's a relatively sophisticated (and risky) trading maneuver that requires a margin account and a keen understanding of the stock market.