Shorting a stock reverses the usual sequence. You borrow shares, sell them, and later buy shares to return to the lender. If the repurchase costs less than the sale, the difference is your gross profit.
The word “borrow” is doing real work. The position depends on both the company's share price and continued access to someone else's shares.
The proceeds and the obligation
Sell 100 borrowed shares at $40 and the sale produces $4,000. If the price falls to $34, buying them back costs $3,400 and the gross profit is $600. If the price rises to $55, repurchasing costs $5,500 and the loss is $1,500.
The $4,000 of proceeds is accompanied by an obligation to return 100 shares. It is not a free cash balance. The SEC's short-sale explanation describes the borrowing process and the asymmetry: a stock can fall only to zero, while its price has no fixed upper bound.
As the price rises, the repurchase obligation grows at the same time your equity falls. A hypothetical $2,000 equity cushion at entry would shrink to $500 after the $1,500 loss, before costs. Meanwhile the short exposure has increased from $4,000 to $5,500. A broker may close the position before you get to see whether the original valuation thesis eventually works.
Borrow creates a daily hurdle
Assume a constant $4,000 stock value and a 12% annual borrow charge using a 360-day convention. Thirty days cost about $40; 90 days cost $120. At a hypothetical 120% annual rate, the same 90 days cost $1,200.
Those are illustrations, not quotations. In practice the charge can depend on the daily share value, the broker's collateral convention, available inventory, and changing rates. Interest on sale proceeds, if any, is a separate line. Ask for the net financing terms rather than treating one displayed percentage as the entire cost.
If your expected price decline is 10%, a $4,000 position has only $400 of expected gross room. A long wait at a high borrow rate can absorb it. Estimating when evidence will emerge is therefore part of valuing the trade, not merely a question of patience.
The lender can change your timetable
Shares can be recalled. If replacement borrowing cannot be arranged, a short may be bought in even though the trader would prefer to remain short. Margin policy can change too: Interactive Brokers' margin-risk disclosure explains its ability to raise requirements or liquidate positions without a prior margin call.
This creates a practical distinction between solvency and staying power. Having a persuasive thesis and adequate wealth elsewhere does not guarantee that the required collateral or borrow will be available in the relevant account at the relevant time.
Dividends and squeezes belong in the model
If the stock pays a dividend while you are short, you generally owe a corresponding payment to the lender. A price decline around the distribution is therefore not a free gain; the cash obligation must be included. The SEC's short-sale guide explains this obligation.
A crowded short can also become self-reinforcing in the wrong direction. Rising prices pressure some shorts to repurchase; those purchases add demand and may push prices higher. “Days to cover,” calculated from short interest and trading volume, is a rough crowding measure. It is not a countdown or a promise that the position can be closed in that many days.
Different ways to take a short view
A put, a short stock position, and a short perpetual have different costs and failure points. A purchased put has a premium and expiry. Borrowed shares have lending and recall conditions. A perpetual has funding, a reference-price mechanism, and liquidation rules. None can be compared using direction alone.
If the expected decline is $400 but the likely wait costs $1,200 in borrow, the valuation argument does not rescue the trade. The business can be overvalued without the available short position being attractive.