What is a “margin-call” and how are they enforced?
"Simplest way to answer this is that on margin, one is using borrowed assets and thus there are strings that come with doing that. Thus, if the amount of equity left gets too low, the broker has a legal obligation to close the position which can be selling purchased shares or buying back borrowed shares depending on if this is a long or short position respectively. Investopedia has an example that they walk through as the call is where you are asked to either put in more money to the account or the position may be closed because the broker wants their money back. What is Maintenance Margin? A maintenance margin is the required   amount of securities an investor must hold in his account if he either   purchases shares on margin, or if he sells shares short. If an   investor's margin balance falls below the set maintenance margin, the   investor would then need to contribute additional funds to the account   or liquidate stocks in the account to bring the account back to the   initial margin requirement. This request is known as a margin call. As discussed previously, the Federal Reserve Board sets the initial   margin requirement (currently at 50%). The Federal Reserve Board also   sets the maintenance margin. The maintenance margin, the amount of   equity an investor needs to hold in his account if he buys stock on   margin or sells shares short, is 25%. Keep in mind, however, that this   25% level is the minimum level set, brokerage firms can increase, but   not decrease this level as they desire. Example: Determining when a margin call would occur. Assume that an   investor had purchased 500 shares of Newco's stock. The shares were   trading at $50 when the transaction was executed. The initial margin   requirement on the account was 70% and the maintenance margin is 30%.   Assume no transaction costs. Determine the price at which the investor   will receive a margin call. Answer: Calculate the price as follows: $50 (1- 0.70) = $21.43 1 - 0.30 A margin call would be received when the price of Newco's stock fell   below $21.43 per share. At that time, the investor would either need   to deposit additional funds or liquidate shares to satisfy the initial   margin requirement. Most people don't want ""Margin Calls"" but stocks may move in unexpected ways and this is where there are mechanisms to limit losses, especially for the brokerage firm that wants to make as much money as possible. Cancel what trade? No, the broker will close the position if the requirement isn't kept. Basically think of this as a way for the broker to get their money back if necessary while following federal rules. This would be selling in a long position or buying in a short sale situation. The Margin Investor walks through an example where an e-mail would be sent and if the requirement isn't met then the position gets exited as per the law." "If you don't have a margin account, then you will not have margin calls. You need a margin account if you wish to ""buy on margin"", to sell stocks ""short"", or to sell options, or maybe some other esoteric things I have not thought of.  If you don't do those things, then you do not need a margin account and will not get margin calls. In your example, it doesn't sound like margin has been used, If you deposit $20 and used it to buy $20 of stock and it then falls to $5, ""they"" did not lose the money, you did.  But if no margin was used, then no margin call would result."
I think George's answer explains fairly well why the brokerages don't allow this - it's not an exchange rule, it's just that the brokerage has to have the shares to lend, and normally those shares come from people's margin, which is impossible on a non-marginable stock. To address the question of what the alternatives are, on popular stocks like SIRI, a deep In-The-Money put is a fairly accurate emulation of an actual short interest.  If you look at the options on SIRI you will see that a $3 (or higher) put has a delta of -$1, which is the same delta as an actual short share. You also don't have to worry about problems like margin calls when buying options.  The only thing you have to worry about is the expiration date, which isn't generally a major issue if you're buying in-the-money options... unless you're very wrong about the direction of the stock, in which case you could lose everything, but that's always a risk with penny stocks no matter how you trade them. At least with a put option, the maximum amount you can lose is whatever you spent on the contract.  With a short sale, a bull rush on the stock could potentially wipe out your entire margin.  That's why, when betting on downward motion in a microcap or penny stock, I actually prefer to use options.  Just be aware that option contracts can generally only move in increments of $0.05, and that your brokerage will probably impose a bid-ask spread of up to $0.10, so the share price has to move down at least 10 cents (or 10% on a roughly $1 stock like SIRI) for you to just break even; definitely don't attempt to use this as a day-trading tool and go for longer expirations if you can. Who runs the courts? Would you have multiple competing courts? Courts usually work on the principle that one party doesn't want to be there; how would that work in a nationless world? What happens when a private enforcement agency gets a monopoly and then wants your stuff?  States work because we have representation, political recourse if there's abuse of the system (in theory).  A 'private enforcement agency' sounds like a fast track to a protection racket.  And also i'd argue that response by the state 100% is what is stopping 99% of people, look at the London 'riots' a few years ago - bunch of kids realized they could get away with it if they all went and looted a place at once, repeatedly happened until the ringleaders got caught or turned in to the authorities.   What right would a private organization have to restrict my freedom in a stateless society? Where does it get the constitutional power? Everything would just devolve into constant warfare between groups - until a single group outperformed the others of course and boom you've got a government again. "With (1), it's rather confusing as to where ""interest"" refers to what you're paying and where it refers to what you're being paid, and it's confusing what you expect the numbers to work out to be. If you have to pay normal interest on top of sharing the interest you receive, then you're losing money. If the lending bank is receiving less interest than the going market rate, then they're losing money. If the bank you've deposited the money with is paying more than the going market rate, they're losing money. I don't see how you imagine a scenario where someone isn't losing money. For (2) and (3), you're buying stocks on margin, which certainly is something that happens, but you'll have to get an account that is specifically for margin trading. It's a specific type of credit with specific rules, and you if you want to engage in this sort of trading, you should go through established channels rather than trying to convert a regular loan into margin trading. If you get a personal loan that isn't specifically for margin trading, and buy stocks with the money, and the stocks tank, you can be in serious trouble. (If you do it through margin trading, it's still very risky, but not nearly as risky as trying to game the system. In some cases, doing this makes you not only civilly but criminally liable.) The lending bank absolutely can lose if your stocks tank, since then there will be nothing backing up the loan." If I sell a covered call, on stock I own 100%, there is no risk of a margin call. The stock goes to zero, I'm still not ask to send in more money. But, if bought on margin, margin rules apply.  A naked put would require you to be able to buy the stock if put to you. As the price of the stock drops, you still need to be able to buy it at the put strike price.  Mark to market is just an expression describing how your positions are considered each day. In fact, buybacks WERE often considered a vehicle for insider trading, especially prior to 1982.  For instance, Prior to the Reagan era, executives avoided buybacks due to fears that   they would be prosecuted for market manipulation. But under SEC Rule   10b-18, adopted in 1982, companies receive a “safe harbor” from market   manipulation liability on stock buybacks if they adhere to four   limitations: not engaging in buybacks at the beginning or end of the   trading day, using a single broker for the trades, purchasing shares   at the prevailing market price, and limiting the volume of buybacks to   25 percent of the average daily trading volume over the previous four   weeks. "Number 2 cannot occur. You can buy the call back and sell the stock, but the broker won't force that #2 choice. To trade options, you must have a margin account. No matter how high the stock goes, once ""in the money"" the option isn't going to rise faster, so your margin % is not an issue.  And your example is a bit troublesome to me. Why would a $120 strike call spike to $22 with only a month left? You've made the full $20 on the stock rise and given up any gain after that. That's all. The call owner may exercise at any time. Edit: @jaydles is right, there are circumstances where an option price can increase faster than the stock price. Options pricing generally follows the Black-Scholes model. Since the OP gave us the current stock price, option strike price, and time to expiration, and we know the risk free rate is <1%, you can use the calculator to change volatility. The number two scenario won't occur, however, because a covered call has no risk to the broker, they won't force you to buy the option back, and the option buyer has no motive to exercise it as the entire option value is time premium." "The SEC regulation on insider trading is ""engage in the purchase and sale of any security on the basis of material non public information"".  The case I remember off the top of my head is Kodak developing an instant photo camera in the 1970s, and about 4 months before Kodak made this knowledge public, the CBOE had to close options trading on Polaroid because they had an excessive buildup of uncovered short positions." Typically the fees are charged when the order is executed.  The only catch I have ever ran into is when an order is partially executed. A good-till-cancel order that gets executed in several blocks over multiple days may get charged a separate commission for each day (but typically not each block).  If this is a simple brokerage account, you could avoid the whole question by using robinhood.com, which charges no commissions or maintenance fees.