How does the person lending shares to the short selller protect themselves if the short sellers are correct?
"Lending of shares happens in the background. Those who have lent them out are not aware that they have been lent out, nor when they are returned. The borrowers have to pay any dividends to the lenders and in the end the borrowers get their stock back. If you read the fine print on the account agreement for a margin account, you will see that you have given the brokerage the permission to silently loan your stocks out.  Since the lending has no financial impact on your portfolio, there's no particular reason to know and no particular protection required.  Actually, brokers typically don't bother going through the work of finding an actual stock to borrow.  As long as lots of their customers have stocks to lend and not that many people have sold short, they just assume there is no problem and keep track of how many are long and short without designating which stocks are borrowed from whom. When a stock becomes hard to borrow because of liquidity issues or because many people are shorting it, the brokerage will actually start locating individual shares to borrow, which is a more time-consuming and costly procedure.  Usually this involves the short seller actually talking to the broker on the phone rather than just clicking ""sell.""" "It is true, as farnsy noted, that you generally do not know when stock that you're holding has been loaned by your broker to someone for a short sale, that you generally consent to that when you sign up somewhere in the small print, and that the person who borrows has to make repay and dividends. The broker is on the hook to make sure that your stock is available for you to sell when you want, so there's limited risk there. There are some risks to having your stock loaned though.  The main one is that you don't actually get the dividend.  Formally, you get a ""Substitute Payment in Lieu of Dividends.""  The payment in lieu will be taxed differently.  Whereas qualified dividends get reported on Form 1099-DIV and get special tax treatment, substitute payments get reported on Form 1099-MISC. (Box 8 is just for this purpose.)  Substitute payments get taxed as regular income, not at the preferred rate for dividends. The broker may or may not give you additional money beyond the dividend to compensate you for the extra tax. Whether or not this tax difference matters, depends on how much you're getting in dividends, your tax bracket, and to some extent your general perspective. If you want to vote your shares and exercise your ownership rights, then there are also some risks.  The company only issues ballots for the number of shares issued by them.  On the broker's books, however, the short sale may result in more long positions than there are total shares of stock. Financially the ""extra"" longs are offset by shorts, but for voting this does not balance. (I'm unclear how this is resolved - I've read that the the brokers essentially depend on shareholder apathy, but I'd guess there's more to it than that.) If you want to prevent your broker from loaning out your shares, you have some options:"
Rich's answer captures the basic essence of short selling with example. I'd like to add these additional points: You typically need a specially-privileged brokerage account to perform short selling.  If you didn't request short selling when you opened your account, odds are good you don't have it, and that's good because it's not something most people should ever consider doing.  Short selling is an advanced trading strategy.  Be sure you truly grok selling short before doing it. Consider that when buying stock (a.k.a. going long or taking a long position, in contrast to short) then your potential loss as a buyer is limited (i.e. stock goes to zero) and your potential gain unlimited (stock keeps going up, if you're lucky!)   Whereas, with short selling, it's reversed: Your loss can be unlimited (stock keeps going up, if you're unlucky!) and your potential gain is limited (i.e. stock goes to zero.)   The proceeds you receive from a short sale – and then some – need to stay in your account to offset the short position.  Brokers require this.  Typically, margin equivalent to 150% the market value of the shares sold short must be maintained in the account while the short position is open. The owner of the borrowed shares is still expecting his dividends, if any.  You are responsible for covering the cost of those dividends out of your own pocket.  To close or cover your short position, you initiate a buy to cover.  This is simply a buy order with the intention that it will close out your matching short position.   You may be forced to cover your short position before you want to and when it is to your disadvantage!  Even if you have sufficient margin available to cover your short, there are cases when lenders need their shares back.  If too many short sellers are forced to close out positions at the same time, they push up demand for the stock, increasing price and deepening their losses.  When this happens, it's called a short squeeze. In the eyes of the public who mostly go long buying stock, short sellers are often reviled. However, some people and many short sellers believe they are providing balance to the market and preventing it sometimes from getting ahead of itself. [Disambiguation: A short sale in the stock market is not related to the real estate concept of a short sale, which is when a property owner sells his property for less than he owes the bank.] Additional references: Here's one option:  Telephone is a lower-tech yet relatively more secure means for transmitting your payment information when a secure web site isn't available. And yet another option:  You could send them an encrypted email, but this would require tools (e.g. GPG), setup (public keys), and expertise on their end which they are unlikely to already have. However, ChrisInEdmonton raised a good point in his comment.  How can you consider them to be a reputable seller when they don't take basic precautions to protect customers' payment information online? The seller may with good faith charge your card the correct amount and deliver the goods that you expect, but how will they protect your credit card information once in their hands?  Would you trust their internal systems if they can't even set up an HTTPS web site? "First, you are not exactly ""giving"" the brokerage $2000. That money is the margin requirement to protect them in the case the stock price rises. If you short 200 shares as in your example and they are holding $6000 from you then they are protected in the event of the stock price increasing to $30/share. Sometime before it gets there the brokerage will require you to deposit more money or they will cover your position by repurchasing the shares for your account. The way you make money on the short sale is if the stock price declines. It is a buy low sell high idea but in reverse. If you believe that prices are going to drop then you could sell now when it is high and buy back later when it is lower. In your example, you are selling 200 shares at $20 and later, buying those at $19. Thus, your profit is $200, not counting any interest or fees you have paid. It's a bit confusing because you are selling something you'll buy in the future. Selling short is usually considered quite risky as your gain is limited to the amount that you sold at initially (if I sell at $20/share the most I can make is if the stock declines to $0).  Your potential to lose is unlimited in theory. There is no limit to how high the stock could go in theory so I could end up buying it back at an infinitely high price. Neither of these extremes are likely but they do show the limits of your potential gain and loss.  I used $20/share for simplicity assuming you are shorting with a market order vs a limit order. If you are shorting it would be better for you to sell at 20 instead of 19 anyway. If someone says I would like to give you $20 for that item you are selling you aren't likely to tell them ""no, I'd really only like $19 for it""" "Concerning the general problem of short selling and the need to borrow shares to complete the transaction : Selling short is a cash transaction.  Unlike a futures contract, where a short seller is entering into a legal agreement to sell something in the future, in the case of short selling a share the buyer of the share is taking immediate delivery and is therefore entitled to all of the benefits and rights that come with share ownership.  In particular, the buyer of the shares is entitled to any dividends payable and, where applicable, to vote on motions at AGMs.  If the short seller has not borrowed the shares to sell, then buyer of non-existent shares will have none of the rights associated with ownership. The cash market is based on the idea of matching buyers and sellers.  It does not accommodate people making promises. Consider that to allow short sellers to sell shares they have not borrowed opens up the possibility of the aggregate market selling more shares than actually exist.  This would lead to all sorts of problematic consequences such as heavily distorting the price of the underlying share.  If everyone is selling shares they have not borrowed willy-nilly, then it will drive the price of the share down, much to the disadvantage of existing share holders. In this case, short sellers who have sold shares they have not already borrowed would be paying out more in dividends to the buyers than the total dividends being paid out by the underlying company. There are instruments that allow for short selling of unowned shares on a futures basis.  One example is a CFD = Contract for Difference.  In the case of CFDs, sellers are obliged to pay dividends to buyers as well as other  costs related to financing. EDIT Regarding your comment, note that borrowing shares is not a market transaction.  Your account does not show you buying a share and then selling it.  It simply shows you selling a share short.  The borrowing is the result of an agreement between yourself and the lender and this agreement is off market.  You do not actually pay the lender for the shares, but you do pay financing costs for the borrowing so long as you maintain your short position. EDIT I realise that I have not actually read your question correctly.  You are not actually talking about ""naked"" short selling.  You are talking about selling shares you already own in a hope of maintaining both a long and short position (gross).  The problem with this approach is that you must deliver the shares to the buyer.  Otherwise, ask yourself what shares is the buyer actually buying if you want the bought shares to remain in your account. If you are not going to deliver your long position shares, then you will need to borrow the shares you are selling short for the reasons I have outlined above." I don't know why a financial investor or a retail trader would do this. But I can guess why a market maker in options would do this. Let us say you buy an option from an option market maker and the market maker sold the option to you. He made a small profit in the bid-ask spread but now he is holding a short position in the option with unlimited risk exposure. So to protect himself, he will take an offsetting position in the underlying and become delta neutral, so that his position is not affected by the moves in the underlying. In the end, he can do this because he is not in the market to make money by betting on direction, unlike the rest of us poor mortals. He is making money from the bid-ask spread. So to ensure that his profits are not eroded by an adverse move in the underlying, he will continuously seek to be delta neutral. But once again, this is for a market maker. For market takers like us, I still don't understand why we would need to delta hedge. If you had an agreement with your friend such that you could bring back a substantially similar car, you could sell the car and return a different one to him.  The nature of shares of stock is that, within the specified class, they are the same.  It's a fungible commodity like one pound of sand or a dollar bill.  The owner doesn't care which share is returned as long as a share is returned.   I'm sure there's a paragraph in your brokerage account terms of service eluding to the possibility of your shares being included in short sale transactions. Investopedia has a good explanation of the term shorting which is what this is. In the simplest of terms, someone is borrowing the bond and selling it with the intent to replace the security and any dividends or coupons in the end. The idea is that if a bond is overvalued, one may be able to buy it back later for a cheaper price and pocket the difference. There are various rules about this including margin requirements to maintain since there is the risk of the security going up in price enough that someone may be forced into a buy to cover in the form of a margin call. If one can sell the bond at $960 now and then buy it back later for $952.38 then one could pocket the difference. Part of what you aren't seeing is what are other bonds doing in terms of their prices over time here. The key point here is that brokers may lend out securities and accrue interest on loaned securities for another point here. ELI5 - you sell something that you don't own with the expectation that it will go down, and then you buy it back when it goes down in order to lock in profit.  You are charged fees to borrow the stock from someone else who currently owns it, and you also run the risk of the market going against you by going up.