Over decades and centuries markets only go up. It is an uphill battle for any short seller, eventually most of them go down in flames. I can name a few billionaire short sellers (Michael Burry, Bill Ackman, David Einhorn, George Soros, John Paulson, Jim Chanos), yet it is a very small club of very capable individuals. Sometimes, some of them are hilariously wrong at their market predictions. (Michael Burry and Nvidia shorts last year - this is as epic of a fail as his correct guess of mortgage market collapse) It is known fact that celebrity short sellers play the market both ways, it is unclear how much of their success can be attributed to shorts and how much of it is due to market's long term tendencies to appreciate in value, boosting their "long" positions; and how much of it is new client inflow. Stanley Druckenmiller admitted in an interview to Bloomberg not so long ago that most of his short deals over the years went so tragic he can't make himself go back in time and check in the books how much he actually lost shorting stocks, so he sort of erases these deals from memory. It is also a difficult game on account of only being allowed to use borrowed funds for short deals. (One has to borrow stocks before shorting, then one pays % for borrowing services - no way around that one) So, on top of making a profit a short trader uses borrowed funds at current 6% annual, which should be deducted from profit. Uphill battle.
It is a mechanism that stems from the reality of big hedge funds, banks, brokerages holding piles of dormant stock. So they lease the stock out to make a penny in addition to pennies they make on each transaction. Some say shorting is anti-social, others see it as correctional mechanism to market's extended stretches of exuberance. The discussion goes on. It's transactional profit, that we know for sure.
If they short Trump into oblivion before election, and they can, since Trump is small fish in the pool of trillions of market capitalisations, well... then he is going to have one difficult election. I think it's a misstep on his part to expose himself to hostile capital in the way he did, going IPO and all, just before critical turning point on his path. He may have convinced himself that his superpowers and charisma will enable him to attract capital from followers, that could be true. But it is equally true that his enemies in Silicon Valley command far greater capital and can squeeze him at will now, through things like short selling.