When shorting a stock, do you pay current market price or the best (lowest) available ask price?
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. In terms of pricing the asset, this functions in exactly the same way as a regular sell, so bids will have to be hit to fill the trade. When shorting an equity, currency is not borrowed; the equity is, so the value of per share liability is equal to it's last traded price or the ask if the equity is illiquid. Thus when opening a short position, the asks offer nothing to the process except competition for your order getting filled. Part of managing the trade is the interest rate risk.  If the asks are as illiquid as detailed in the question, it may be difficult even to locate the shares for borrowing. As a general rule, only illiquid  equities or those in free fall may be temporarily unable for shorting. Interactive Brokers posts their securities financing availabilities and could be used as a proxy guide for your broker. I would never use a market order.  Some brokerages have an approval process your short-sale goes through before going to market.  This can take some time.  So the market prices may well be quite different later. Some brokerages use a separate account for short sales, so you must get their approval for the account before you can do the trade.   I like the listing of shares available for shorting the Interactive Brokers has but I have experienced orders simply going into dead-air and sitting there on the screen, not being rejected, not going to market, not doing anything --- even though the shares are on the list.
It is unlikely that buying 100 shares will have any effect on a stock's price, unless the stock's average trading volume is incredibly low.  That being said, no matter how many share you buy, there's no way to know what the impact on the price will be, because that's only one factor in how shares are priced.  If anyone could figure out the answer to your question then they'd be extremely rich, because they'd simply watch for big share trades and then buy those stocks on the way up. The market makers who actually execute the trades are the ones who set the prices, and most stocks have multiple market makers trading the stock, so the bid/ask you see is the highest bid and lowest ask.  The market makers set the price based on what the trend of the stock is.  If, for instance, there's a large number of sell orders against a stock, the market makers will start dropping the bid prices as they fill execution orders, and as they see buy orders increase, they'll raise ask prices as they fill execution orders.  The market makers earn the difference between what they paid to buy someone's stock who was selling and what they get from someone else who buys it. This is a simplified explanation, so pro traders, don't beat me up! (grin) So, basically, it takes quite a bit of share volume in one direction or another to affect a stock's price.  I can guarantee a 100-share trade wouldn't even be noticed by market makers. I hope this helps. Good luck! "The question you are asking concerns the exercise of a short option position.  The other replies do not appear to address this situation. Suppose that Apple is trading at $96 and you sell a put option with a strike price of $95 for some future delivery date - say August 2016.  The option contract is for 100 shares and you sell the contract for a premium of $3.20. When you sell the option your account will be credited with the premium and debited with the broker commission.  The premium you receive will be $320 = 100 x $3.20.  The commission you pay will depend on you broker. Now suppose that the price of Apple drops to $90 and your option is exercised, either on expiry or prior to expiry.  Then you would be obliged to take delivery of 100 Apple shares at the contracted option strike price of $95 costing you $9,500 plus broker commission.  If you immediately sell the Apple shares you have purchased under your contract obligations, then assuming you sell the shares at the current market price of $90 you would realise a loss of $500 ( = 100x($95-$90) )plus commission.  Since you received a premium of $320 when you sold the put option, your net loss would be $500-$320 = $180 plus any commissions paid to your broker. Now let's look at the case of selling a call option.  Again assume that the price of Apple is $96 and you sell a call option for 100 shares with a strike price of $97 for a premium of $3.60.  The premium you receive would be $360 = 100 x $3.60.  You would also be debited for commission by your broker. Now suppose that the price of Apple shares rises to $101 and your option is exercised.  Then you would be obliged to deliver 100 Apple shares to the party exercising the option at the contracted strike price of $97.  If you did not own the shares to effect delivery, then you would need to purchase those shares in the market at the current market price of $101, and then sell them to the party exercising the option at the strike price of $97.  This would realise an immediate loss of $400 = 100 x ($101-$97) plus any  commission payable.  If you did own the shares, then you would simply deliver them and possibly pay some commission or a delivery fee to your broker.  Since you received $360 when you sold the option, your net loss would be $40 = $400-$360 plus any commission and fees payable to the broker. It is important to understand that in addition to these accounting items, short option positions carry with them a ""margin"" requirement.  You will need to maintain a margin deposit to show ""good faith"" so long as the short option position is open.  If the option you have sold moves against you, then you will be called upon to put up extra margin to cover any potential losses." "Futures are an agreement to buy or sell something in the future.  The futures ""price"" is the price at which you agree to make the trade.  This price does not indicate what will happen in the future so much as it indicates the cost of buying the item today and holding it until the future date.  Hence, for very liquid products such as stock index futures, the futures price is a very simple function of today's stock index value and current short-term interest rates.  If the stock exchange is closed but the futures exchange is open, then using the futures price and interest rates one can back out an implied ""fair value"" for the index, which is in essence the market's estimate of what the stock index value would be right now if the stock market were open.  Of course, as soon as the stock exchange opens, the futures price trades to within a narrow band of the actual index value, where the size of the band depends on transaction costs (bid-ask spread, commissions, etc.)." What financial instruments are there that are profitable when an underlying assets falls? The instrument you are looking for is called an Option, specifically a Put Option. It allows you, within the validity date, to sell ('Put') the respective shares to the option giver, at the predefined Strike Price. For example, let's assume APPL trades currently at 100 $ per share, and you think they will go down a lot. You buy one Put Option for 100 shares (they always come for larger amounts like 100s) for a Strike Price of 90 $, and pay 5 $ for it (it would be cheap if nobody believes they will fall that much). Note the last sentence under 2. - it is rather easy and very common when trading options to make complete losses. You have been warned. Are they available for IPOs? They could be available for IPOs, even before the IPO. However, someone has to put them out (some large bank, typically), which is some effort, and they would only do that if they expect enough interest and volume in the trade. most of the time, there will be no such options on the market. Are they available for foreign stocks?Yes, but again only selectively - only if the stock is well known and interesting enough for a broad audience. 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. Your three options are: Options 2 and 3 are obviously identical (other than transaction costs), so if you want to keep the stock, go for option 1, otherwise, go for option 3 since you have the same effect as option 2 with no transaction costs.  The loss will likely also offset some of the other short term gains you mentioned. The Art of Short Selling by Kathryn Stanley providers for many case studies about what kind of opportunities to look for from a fundamental analysis perspective. Typically things you can look for are financing terms that are not very favorable (expensive interest payments) as well as other constrictions on cash flow, arbitrary decisions by management (poor management), and dilution that doesn't make sense (usually another product of poor management). From a quantitative analysis perspective, you can gain insight by looking at the credit default swap rate history, if the company is listed in that market. The things that affect a CDS spread are different than what immediately affects share prices. Some market participants trade DOOMs over Credit Default Swaps, when they are betting on a company's insolvency. But looking at large trades in the options market isn't indicative of anything on its own, but you can use that information to help confirm your opinion. You can certainly jump on a trend using bad headlines, but typically by the time it is headline news, the majority of the downward move in the share price has already happened, or the stock opened lower because the news came outside of market hours. You have to factor in the short interest of the company, if the short interest is high then it will be very easy to squeeze the shorts resulting in a rally of share prices, the opposite of what you want. A short squeeze doesn't change the fundamental or quantitative reasons you wanted to short. The technical analysis should only be used to help you decide your entry and exit price ranges amongst an otherwise random walk. The technical rules you created sound like something a very basic program or stock screener might be able to follow, but it doesn't tell you anything, you will have to do research in the company's public filings yourself. Depends on when you are seeing these bids & asks-- off hours, many market makers pull their bid & ask prices entirely.  In a lightly traded stock there may just be no market except during the regular trading day.