You've got a number of misconceptions here. Let's make up some numbers.
- Alice and Dave both have 100 shares of XYZ that they bought for $10 each.
- Bob has $15.
- Carol has $1000.
- XYZ is trading at $10 today.
- Bob makes a deal with Alice: loan me 100 shares of XYZ today and I will give them back to you tomorrow, plus my fifteen bucks. (See below)
- Alice thinks "I can't lose on this deal; I'm going to make a 1.5% return on my investment!"
- Bob takes the shares from Alice and sells them to Carol for $10 each.
- Now Alice has nothing, Bob has $1015, Carol and Dave have 100 shares.
- The next day, the price of XYZ has fallen to $9.
- Bob offers to buy Dave's shares for $9 each and Dave agrees.
- Bob now has 100 shares and $115.
- Bob returns the shares to Alice and pays her the $15.
- Bob now has $100. Alice has 100 shares and $15. Carol has 100 shares. Dave has $900.
- Bob started with $15 and ended up with $100 one day later. So Bob is up 567% in one day. Alice is down 8.5%. Carol and Dave are both down 10%.
Looks great for Bob!
Now run the math and see what happens if Bob guesses wrong and the price of XYZ goes up a dollar instead of down a dollar. Leverage works both ways.
Note that things are also good for Alice. She's long in a stock that declined in value, but her fee to Bob decreased her loss; that's pure profit.
The figure of "and fifteen bucks" for the interest is unreasonably high if the sale really is happening "tomorrow"; for pedagogical reasons I'm exaggerating. In reality the interest on the loan of a heavily traded stock would be in the ballpark of 3% or 4% per year, and some of that would go to the broker who brokered this deal.
What happens if at point 5, Person C does not want to sell shares back to Person B?
Carol doesn't sell back to Bob. Dave sells to Bob.
And even if Person B tries to buy from other people other than Person C, they all say no.
The supposition of the exercise is that the price of XYZ went down. If no one is willing to sell then the price did not go down. If no one is willing to sell then the price went up, and we have not yet determined what the price is. The price is determined exactly when someone is willing to sell at the same price that someone else is willing to buy!
Seriously, do the exercise again and see what happens if the price goes up instead of down. Bob ends up in a bad way. Short selling is dangerous.
How does Bob find Alice?
Now of course in practice, Bob probably doesn't call up Alice directly. Bob calls up his broker, and the broker figures out who is willing to lend shares, and who will get what cut of the fee.
How that works can vary, but typically it goes like this.
We know that a short sale is: Bob borrows stock, sells it now, buys it later, pays the stock back plus interest. That's not the only way to trade on a borrowed asset. Consider the following scenario:
Alice has $1000. She believes XYZ is going to go way up tomorrow, so she buys 100 shares for $10 each. If it goes up to $11 tomorrow, she sells and now she has $1100, for a total profit of 10%. But then Alice has a brilliant idea. She asks her broker "Can I borrow $1000 from you with my 100 shares of XYZ as collateral?" Alice takes the $1000 and buys 100 more shares for $10. Now if the stock goes up to $11, Alice sells her 200 shares for $2200, pays back the $1000 she owes, pays a couple bucks in interest on the loan, and now she has, say, $1198. She just doubled her profit for $2!
This strategy is called buying on margin, and like short selling it is super dangerous because, well, run the numbers again if the stock goes down instead of up.
Alice and Bob are therefore made for each other. They're both gambling with borrowed assets; Alice is gambling with borrowed money and betting the stock will go up; Bob is gambling with borrowed stock and betting it will go down. One of them will be wrong and get totally screwed.
So Alice's broker makes a deal with Alice: "I'll lend you the $1000 under the following conditions: (1) you pay me back with interest, (2) if XYZ goes down a lot, I get to sell your XYZ on your behalf, to ensure that you don't go bankrupt before you pay me back the $1000 you owe me, (3) I get to lend your XYZ to my buddy Bob the short seller over here, and I'll give you a cut of the fee that I'm going to charge him".
So when you say
Bob borrows 100 shares from Alice without Alice's knowledge (because that's how short selling is supposed to work?)
that's not right. Alice agreed to possibly lend to Bob when Alice opened a margin trading account, and Alice is getting a cut of the interest when that happens.
Of course all of this is super dangerous and there are additional restrictions in place to make sure that margin buyers and short sellers don't default on their loans. Bob's broker will not let Bob get away with selling short with $15 in his account; Bob's broker knows that Bob will need to sell assets to cover the short position if things go wrong for Bob. Bob's got to have a whole lot more than that $15 in order to make a short sale.
EXERCISE: Run the numbers on our original scenario again, this time with Alice having 200 shares of XYZ and owing her broker $1000. Try running the scenario where XYZ goes up, and where XYZ goes down, and see how much Alice and Bob end up ahead or behind.
Trading with borrowed assets is called leverage because its like pushing on the small end of a lever: a small movement on the small end translates into a big movement at the big end. But leverage works both ways; a small movement in the wrong direction leads to a big bad outcome for you.
Some follow-up questions:
What is the price of a stock if no one is willing to sell?
There is no price. The price of a thing is only defined when there is a seller and a buyer and they agree on a price.
We can make estimates of a price -- or, less charitably, guesses -- of varying qualities depending on the information we have available.
When a stock price is quoted, that's an estimate of the current price simply computed by taking the price of the most recent transaction. Odds are pretty good that the next transaction will be at a similar price.
You can also get quotes of the current "ask price" -- that is the highest price where a seller says "I'd be willing to sell at this price". And you can also get the "bid price", which is the lowest price that any buyer is willing to buy at. Normally they are very close together, and when someone budges, the trade happens.
You can also get an idea of the price of a thing by looking at all the shares that everyone wants to sell and the price they'd sell at, and similarly for buyers. That's called a "depth chart" and you can learn a lot about the demand for a stock by studying it. There are many tutorials on the internet that will show you how to read a depth chart.
What if someone sells short and then the price goes way up, so high that the short seller cannot afford to buy back the stock?
Then the short seller is bankrupt, and the person they borrowed the stock from is a creditor who has a claim. A bankruptcy court will determine how the assets of the short seller are distributed amongst the creditors.
Of course in practice this does not happen too often. If you are doing a short sale through a broker and the price started to shoot up then they would probably require you to buy back while you still can, to cover your obligation. There are many mechanisms in place designed to ensure that irresponsible short selling does not bankrupt people. They don't always work.
When you loan out anything, whether its a stock or cash, you run the risk of not being paid back. That's called counterparty risk. (There are a variety of things you can do to mitigate counterparty risk -- do some research on Credit Default Swaps, for instance.) But in general, the reason why you justify charging interest for the loan is to cover the risk that you won't get paid back. If you make more on interest from people who do pay you back than you lose on defaults, you're ahead.
What if I am very wealthy and I successfully buy all of a stock. Can I then charge the short sellers a price of my choice, or bankrupt them?
Yep.
In my hypothetical situation, because I now own ALL of XYZ, that means creditor Alice who loaned XYZ to the short seller Bob lost their stocks? And they accepted that risk in exchange for interest from Bob?
That's correct. Look at our scenario again:
- Alice and Dave both have 100 shares of XYZ that they bought for $10 each.
- Bob has $15.
- Carol has $1000.
- XYZ is trading at $10 today.
- Bob makes a deal with Alice: loan me 100 shares of XYZ today and I will give them back to you tomorrow, plus my fifteen bucks.
- Bob takes the shares from Alice and sells them to Carol for $10 each.
- Now Alice has nothing, Bob has $1015, Carol and Dave have 100 shares.
- The next day, John buys Carol and Dave's shares for $20 each, which they gladly accept.
- John buys up everyone else's XYZ for whatever they want to be paid, outbidding everyone else.
- Alice, who would love to sell to John for $20 calls up Bob and says hey buddy, I'd like those shares of XYZ and my $15 back RIGHT NOW please.
- Bob has no ability to buy XYZ at any price because John is outbidding him.
- At the end of the day, Bob has $1015, Alice has empty promises, Carol and Dave have $2000, and John just spent twice the previous day's market price in order to gain control of XYZ. Bob owes Alice $15, which he can repay, and 100 shares of XYZ, which he cannot. Alice and Bob are going to court.
Of course this scenario is extremely rare in real life.
If these sorts of shenanigans interest you, consider watching the classic comedy "Trading Places". It is about controlling a market, but not in order to screw over short sellers. Rather, it's about using psychological tricks and insider information to manipulate the price of frozen concentrated orange juice futures. I believe there was an episode of Planet Money that dissected the (now illegal) techniques used in that movie.