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Why does short selling require borrowing?
Selling short is simply by definition the selling, then later re-buying of stock you don't initially own. Say you tally your entire portfolio balance: the quantity of each stock you own, and your cash assets. Let's call this your "initial position". We define "profit" as any increase in assets, relative to this initial position. If you know a particular stock will go down, you can realize a profit by selling some of that stock, waiting for the price to go down, then buying it back. In the end you will have returned to your initial position, except you will have more cash. If you sell 10 shares of a stock valued at £1.50, then buy them back at £1.00, you will make a £5.00 profit while having otherwise returned to your previous position. If you do the same, but you initially owned 1000 shares, sold just 10 of those, then bought 10 back, that's still a profit of £5.00. Selling short is doing the same thing, but with an initial and ending balance of 0 shares. If you initially own 0 shares, sell 10, then buy 10 back, you return to your initial position (0 shares) plus a profit of £5.00. (And in practice you must also pay a borrowing fee to do this.) The advantage of selling short is it can be done with any stock, not just those currently owned.