Betting on the Downside

Short selling is the practice of borrowing shares and selling them, hoping to buy them back later at a lower price. If a stock falls from $100 to $70, a short seller profits $30 per share (minus borrowing costs). If the stock rises, the short seller loses money.

The Mechanics of a Short Sale

Borrow shares — You borrow shares from your broker (which typically sources them from other clients' margin accounts, with permission). You pay a borrowing fee — low for easy-to-borrow stocks, high for hard-to-borrow stocks (5%, 20%, even 100%+ annually).
Sell borrowed shares — You sell them in the open market at the current price. The cash proceeds go into your account but are held as collateral.
Buy back (cover) — You eventually buy the same number of shares in the open market and return them to the lender. If the price has fallen, you profit. If it's risen, you lose.

The Risks: Why Short Selling Is Dangerous

Unlimited downside — When you buy a stock (go long), the most you can lose is 100% of your investment. When you short a stock, your potential loss is theoretically unlimited — there's no ceiling on how high a stock can go. A $10,000 short position on a stock that doubles costs you $10,000. If it triples, you lose $20,000. If it goes 10x... you can lose far more than you put up.
Short squeeze — When a heavily shorted stock starts rising, short sellers are forced to buy back shares to limit losses, which pushes the price even higher, forcing more short sellers to cover — a vicious cycle. The GameStop squeeze of January 2021 (stock rose from $17 to $483) wiped out multi-billion-dollar hedge funds in weeks.
Borrowing costs — You pay interest to borrow shares. The harder to borrow (more short interest), the higher the fee. These costs eat into profits and accumulate daily. A short position held for months can lose money even if the stock doesn't move, purely from borrowing fees.
Dividend payments — If the stock pays a dividend while you're short, you owe that dividend to the lender. Short sellers pay dividends, they don't collect them.
Buy-ins — The lender can demand their shares back at any time. If your broker can't find replacement shares to borrow, your position is forcibly closed at the current market price — potentially at a terrible time.

The Constructive Role of Short Sellers

Despite the controversy, short sellers serve an important market function: they identify fraud, overvaluation, and deteriorating businesses that the market has overlooked. Famous short sellers have exposed accounting frauds (Enron, Wirecard, Luckin Coffee) and overhyped companies long before the market caught on. Short sellers are the market's immune system — uncomfortable but necessary.

Key Takeaways

Short selling = borrow shares, sell them, buy back later at (hopefully) a lower price.
Risk is theoretically unlimited. Short squeezes can destroy accounts in days.
Short selling is not suitable for beginners or most retail investors. The asymmetry of risk makes it dangerous.