Can I cover a short sale with the stock I already own?
Yes, you call the broker and tell him to use those shares to deliver to the short position. Yes you can.  This is known as a short selling against the box.  In the old days, this was used to delay a taxable event.  You could lock in a gain without triggering a taxable event.  Any loss on one side of the box would be offset by a loss on the other side, and vice versa.  However, the IRS clamped down on this, and you will realize the gain on your long position as soon as you go short on the other side.  See http://www.investopedia.com/terms/s/sellagainstthebox.asp. As to how to initiate the short cover, just transfer the long position to the same account as your short position and make sure your broker covers the short.  Should be relatively easy.
In the money puts and calls are subject to automatic execution at expiration. Each broker has its own rules and process for this.  For example, I am long a put. The strike is $100. The stock trades at the close, that final friday for $90. I am out to lunch that day. Figuratively, of course. I wake up Saturday and am short 100 shares. I can only be short in a margin account. And similarly, if I own calls, I either need the full value of the stock (i.e. 100*strike price) or a margin account.  I am going to repeat the key point. Each broker has its own process for auto execution. But, yes, you really don't want a deep in the money option to expire with no transaction.  On the flip side, you don't want to wake up Monday to find they were bought out by Apple for $150. why can't I just use the same trick with my own shares to make money on the way    down? Because if you sell shares out of your own portfolio, by definition, you are not selling short at all. If you sell something you own (and deliver it) - then there is no short involved. A short is defined as a net negative position - i.e. you sell shares you do not have. Selling shares you own is selling shares you own - no short involved. You must borrow the shares for a short because in the stock market, you must DELIVER. You can not deliver shares you do not own. The stock market does not work on promises - the person who bought the shares expects ownership of them with all rights that gives them. So you borrow them to deliver them, then return them when you buy them back. It's not quite identical, due to fees, stock rights, and reporting & tax obligations.  But the primary difference is that a person could have voting rights in a company while maintaining zero economic exposure to the company, sometimes known as empty voting. As an abstract matter, it's identical in that you reduce your financial exposure whether you sell your stock or short it.  So the essence of your question is fundamentally true.  But the details make it different.  Of course there are fee differences in how your broker will handle it, and also margin requirements for shorting.  Somebody playing games with overlapping features of ownership, sales, and purchases, may have tax and reporting obligations for straddles, wash sales, and related issues.  A straight sale is generally less complicated for tax reporting purposes, and a loss is more likely to be respected than someone playing games with sales and purchases. But the empty voting issue is an important difference.  You could buy stock with rights such as voting, engage in other behavior such as forwards, shorts, or options to negate your economic exposure to the stock, while maintaining the right to vote.  Of course in some cases this may have to be disclosed or may be covered by contract, and most people engaging in stock trades are unlikely to have meaningful voting power in a public company.  But the principle is still there. As explained in the article by Henry Hu and Bernie Black: Hedge funds have been especially creative in decoupling voting rights from economic ownership. Sometimes they hold more votes than economic ownership - a pattern we call empty voting. In an extreme situation, a vote holder can have a negative economic interest and, thus, an incentive to vote in ways that reduce the company's share price. Sometimes investors hold more economic ownership than votes, though often with morphable voting rights - the de facto ability to acquire the votes if needed. We call this situation hidden (morphable) ownership because the economic ownership and (de facto) voting ownership are often not disclosed. "You didn't win in case B. Borrowing shares and then selling them is known as ""selling short"".  You received $2000 when you sold short 100 shares at $20.  You spent $1000 to buy them back at $10, so you come out $1000 ahead on that deal.  But at the same time, the 100 shares you already owned have declined in value from $20 to $10, so you are down $1000 on that deal.  So you've simply broken even, and you are still out the interest and transaction fees. In effect, a short sale allows you to sell shares you don't own.  But if you do already own them, then the effect is the same as if you just sold your own shares.  This makes it easier to see that this is just a complicated and expensive way of accomplishing nothing at all." It seems also on some international markets this is allowed. http://www.businessinsider.com/li-hejun-shorting-hanergy-2015-5 "If the owner of the stock wants it back, they ""call"" it back. There are no guarantees of how long you can keep it for your short, or the cost involved to hold it.  Usually, everyone knows about a particular set-up (e.g. a warrant or convertible bond mispricing) that is attractive for arbitrage. This causes the associated stock to be in high demand thus expensive to borrow for shorting, or impossible to find for any price at all." "This doesn't make sense to me. Writing a covered call gives him a long delta position - the exact opposite of what he wants. And the tax losses won't turn a losing position into a winning one.  Is there something I'm missing?  Edit: Also, doesn't ""shorting against the box"" mean he has to have a long position, and short against that? That means you've got zero net delta, which isn't very useful at all..." When you want to short a stock, you are trying to sell shares (that you are borrowing from your broker), therefore you need buyers for the shares you are selling.  The ask prices represent people who are trying to sell shares, and the bid prices represent people who are trying to buy shares. Using your example, you could put in a limit order to short (sell) 1000 shares at $3.01, meaning that your order would become the ask price at $3.01.  There is an ask price ahead of you for 500 shares at $3.00.  So people would have to buy those 500 shares at $3.00 before anyone could buy your 1000 shares at $3.01. But it's possible that your order to sell 1000 shares at $3.01 never gets filled, if the buyers don't buy all the shares ahead of you.  The price could drop to $1.00 without hitting $3.01 and you will have missed out on the trade. If you really wanted to short 1000 shares, you could use a market order.  Let's say there's a bid for 750 shares at $2.50, and another bid for 250 shares at $2.49.  If you entered a market order to sell 1000 shares, your order would get filled at the best bid prices, so first you would sell 750 shares at $2.50 and then you would sell 250 shares at $2.49. I was just using your example to explain things.  In reality there won't be such a wide spread between the bid and ask prices.  A stock might have a bid price of $10.50 and an ask price of $10.51, so there would only be a 1 cent difference between putting in a limit order to sell 1000 shares at $10.51 and just using a market order to sell 1000 shares and getting them filled at $10.50. Also, your example probably wouldn't work in real life, because brokers typically don't allow people to short stocks that are trading under $5 per share. As for your question about how often you are unable to make a short sale, it can sometimes happen with stocks that are heavily shorted and your broker may not be able to find any more shares to borrow. Also remember that you can only short stocks with a margin account, you cannot short stocks with a cash account.