How can I find out who the major short sellers are in a stock?
There is no way to know anything about who has shorted stuff or how concentrated the positions are in a few investors.  Short positions are not even reported in 13(F) institutional filings. I'll take the bonus points, though, and point you to the US Equity Short Interest data source at quandl.
"Market makers, traders, and value investors would be who I'd suspect for buying the stock that is declining. Some companies stocks can come down considerably which could make some speculators buy the stock at the lower price thinking it may bounce back soon. ""Short sellers"" are out to sell borrowed stocks that if the stock is in free fall, unless the person that shorted wants to close the position, they would let it ride. Worthless stocks are a bit of a special case and quite different than the crash of 1929 where various blue chip stocks like those of the Dow Jones Industrials had severe declines. Thus, the companies going down would be like Apple, Coca-Cola and other large companies that people would be shocked to see come down so much yet there are some examples in recent history if one remembers Enron or Worldcom. Stocks getting delisted tend to cause some selling and there are some speculators may buy the stock believing that the shares may be worth something only to lose the money possibly as one could look at the bankrupt cases of airlines and car companies to study some recent cases here. Circuit breakers are worth noting as these are cases when trading may be halted because of a big swing in prices that it is believed stopping the market may cause things to settle down." "I'm adding to @Dilip's basic answer, to cover the additional points in your question.  I'll assume you are referring to publicly traded stock options, such as those found on the CBOE, and not an option contract entered into privately between two specific counterparties (e.g. as in an employer stock option plan). Since you are not obligated to exercise a call option you purchased on the market, you don't need to maintain funds on account for possible exercising.  You could instead let the option expire, or resell the option, neither of which requires funds available for purchase of the underlying shares. However, should you actually choose to exercise the call option (and usually this is done close to expiration, if at all), you will be required to fund your account much like if you bought the underlying shares in the first place.  Call your broker to determine the exact rules and timing for when they need the money for a call-option exercise. And to expand on the idea of ""cancelling"" an option you purchased: No, you cannot ""cancel"" an option contract, per se.  But, you are permitted to sell the call option to somebody else willing to buy, via the market.  When you sell your call option, you'll either make or lose money on the sale – depending on the price of the underlying shares at the time (are they in- or out- of the money?), volatility in the market, and remaining time value.  Once you sell, you're back to ""no position"".  That's not the same as ""cancelled"", but you are out of the trade, whether at profit or loss. Furthermore, the option writer (i.e. the seller who ""sold to open"" a position, in writing the call in the first place) is also not permitted to cancel the option he wrote.  However, the option writer is permitted to close out the original short position by simply buying back a matching call option on the market.  Again, this would occur at either profit or loss based on market prices at the time.  This second kind of buy order – i.e. made by someone who initially wrote a call option – is called a ""buy to close"", meaning the purchase of an offsetting position.  (The other kind of buy is the ""buy to open"".) Then, consider: Since an option buyer is free to re-sell the option purchased, and since an option writer (who ""sold to open"" the new contract) is also free to buy back an offsetting option, a process known as clearing is required to match remaining buyers exercising the call options held with the remaining option writers having open short positions for the contract.  For CBOE options, this clearing is performed by the Options Clearing Corporation.  Here's how it works (see here): What is the OCC? The Options Clearing Corporation is the sole issuer of all securities   options listed at the CBOE, four other U.S. stock exchanges and the   National Association of Securities Dealers, Inc. (NASD), and is the   entity through which all CBOE option transactions are ultimately   cleared. As the issuer of all options, OCC essentially takes the   opposite side of every option traded. Because OCC basically becomes   the buyer for every seller and the seller for every buyer, it allows   options traders to buy and sell in a secondary market without having   to find the original opposite party.   [...]   [emphasis above is mine] When a call option writer must deliver shares to a call option buyer exercising a call, it's called assignment.  (I have been assigned before, and it isn't pleasant to see a position called away that otherwise would have been very profitable if the call weren't written in the first place!) Also, re: ""I know my counter party cannot sell his shares"" ... that's not strictly true.  You are thinking of a covered call.  But, an option writer doesn't necessarily need to own the underlying shares.  Look up Naked call (Wikipedia).  Naked calls aren't frequently undertaken because a naked call ""is one of the riskiest options strategies because it carries unlimited risk"".  The average individual trader isn't usually permitted by their broker to enter such an order, but there are market participants who can do such a trade. Finally, you can learn more about options at The Options Industry Council (OIC)." The majority of honest sellers who are serious about their business and committed to the Amazon platform just authorize all returns/refunds as quick as they can because they don't want to risk having a bad rating/review punctuate their profile. This policy does not change much for any of those sellers, just a few less clicks they need to make every day.  Agreed that many smaller shops or those who are just in it for the short term don't have the room to absorb the costs of meeting the same level of service Amazon provides directly -- but shouldn't we all aspire to it anyway? This is a gross simplification as there are a few different ways to do this. The principle overall is the same though. To short a stock, you borrow X shares from a third party and sell them at the current price. You now owe the lender X shares but have the proceeds from the sale. If the share price falls you can buy back those shares at the new lower price, return them to the lender and pocket the difference. The risk comes when the share price goes the other way, you now owe the lender the new value of the shares, so have to find some way to cover the difference. This happened a while back when Porsche made a fortune buying shares in Volkswagen from short sellers, and the price unexpectedly rose. Exact rules may be different depending on the size of the investor, the specific broker, and the country.  For both the US and Canada, short sales occur only through one's margin account.  And shares that are borrowed for shorting only come from a margin account. Shares held in a cash account are not available for shorting.  From Wikipedia Short (finance) - The speculator instructs the broker to sell the shares and the   proceeds are credited to his broker's account at the firm upon which   the firm can earn interest. Generally, the short seller does not earn   interest on the short proceeds and cannot use or encumber the proceeds   for another transaction. As with many questions, I'd suggest you contact your broker for the exact details governing your account. At the bottom of the page you linked to, NASDAQ provides a link to this page on nasdaqtrader.com, which states Each FINRA member firm is required to report its “total” short interest positions in all customer and proprietary accounts in NASDAQ-listed securities twice a month. These reports are used to calculate short interest in NASDAQ stocks. FINRA member firms are required to report their short positions as of settlement on (1) the 15th of each month, or the preceding business day if the 15th is not a business day, and (2) as of settlement on the last business day of the month.* The reports must be filed by the second business day after the reporting settlement date. FINRA compiles the short interest data and provides it for publication on the 8th business day after the reporting settlement date. The dates you are seeing are the dates the member firms settled their trades. In general (also from nasdaq.com), the settlement date is The date on which payment is made to settle a trade. For stocks traded on US exchanges, settlement is currently three business days after the trade. http://www.euroinvestor.com/exchanges/nasdaq/macromedia-inc/41408/history will work as DumbCoder states, but didn't contain LEHMQ (Lehman Brother's holding company). You can use Yahoo for companies that have declared bankruptcy, such as Lehman Brothers: http://finance.yahoo.com/q/hp?s=LEHMQ&a=08&b=01&c=2008&d=08&e=30&f=2008&g=d but you have to know the symbol of the holding company. "The shares available to short are a portion of those shares held by the longs. This number is actually much easier to determine outside of active trading hours, but either way doesn't really impact the matter at hand since computers are pretty good at counting things. If your broker is putting up obstacles to your issuing sell short limit orders in the pre-market then there is likely some other reason (maybe they reserve that function to ""premium"" account holders?)"