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Why does short selling require borrowing?
A simple way to ask the question might be to say "why can't I just use the same trick with my own shares to make money on the way down? Why is borrowing someone else's shares necessary to make the concept a viable one? Why isn't it just the inverse of 'going long'?" A simple way to think about it is this: to make money by trading something, you must buy it for less than you sell it for.  This applies to stocks like anything else.  If you believe the price will go up, then you can buy them first and sell them later for a higher price.  But if you believe the price will go down, the only way to buy low and sell high is to sell first and buy later.  If you buy the stock and it goes down, any sale you make will lose you money. I'm still not sure I fully understand the point of your example, but one thing to note is that in both cases (i.e., whether you buy the share back at the end or not), you lost money.  You say that you "made $5 on the share price dropping", but that isn't true at all: you can see in your example that your final account balance is negative in both cases.  You paid $20 for the shares but only got $15 back; you lost $5 (or, in the other version of your example, paid $20 and got back $5 plus the depreciated shares). If you had bought the shares for $20 and sold them for, say, $25, then your account would end up with a positive $5 balance; that is what a gain would look like.  But you can't achieve that if you buy the shares for $20 and later sell them for less. At a guess, you seem to be confusing the concept of making a profit with the concept of cutting your losses.  It is true that if you buy the shares for $20 and sell them for $15, you lose only $5, whereas if you buy them for $20 and sell for $10, you lose the larger amount of $10.  But those are both losses.  Selling "early" as the price goes down doesn't make you any money; it just stops you from losing more money than you would if you sold later.