> who do you make money from?

When you own stock at a broker in a margin account, you may sign an agreement to allow the broker to lend out your stock to someone else. For lending your stock, you are entitled to a stock-borrow fee which usually is quite small say 0.25%, and paid by the borrower (short-seller). The borrower then sells the stock to someone else. At a later point, the short seller closes their position by buying it back, and returning it to you. This is roughly the mechanics of it. So, to answer your question, the short seller makes money from folks who buy high and sell low. In this specific example, the stock-borrow fee say was 5% because, the float is still low, and if the short seller borrowed at $165 after the IPO and sold it, and then bought it back at $135 and closed their position, they made money from folks who bought at $165 and sold at $135.

Alice holds SpaceX stock and believes it will rise. Bob believes the stock will fall. Alice and Bob reach an agreement for Alice to "lend" their SpaceX stock to Bob for a small "fee". Bob immediately sells the SpaceX stock at the current market value. After some time Bob will buy back the sold SpaceX stock at the current market value (hopefully less than Bob sold it for) and return the "borrowed" SpaceX stock to Alice thereby fulfilling the original contract.

It's also possible Bob's thesis on SpaceX could have been wrong and the shares could skyrocket. There's usually a provision in the contract for Alice to recall the shares she lent to Bob. In this case, Bob would be forced to buy SpaceX stock at the current market value and likely lose money on the overall trade.

To answer your specific question, "Who do you make money from?" It's actually not clear. Bob selling-high and buying-low doesn't necessarily mean whom Bob sells-to and whom he buys-from are on losing sides of the trade despite Bob making a profit. E.g. the buyer of Bob's short-sell could write calls and the stock could close pass the strike on expiration and turn a small profit as well.

Keep in mind that unlike purchasing a stock where the most amount of money you can lose is the amount of money you spend buying the stock (assuming you didn't buy it on margin), if you directly short a stock, there's technically no limit to the amount of money you could lose. If a stock goes up 1000% after you short it, then you could lose far more money than you put into it.

Someone who bought and expected to make a profit, but reached a point where they hit their stop loss or just wanted to get out the trade at any cost and couldn’t bear to wait longer. Quite possible a redditor who frequents r/wallstreetbets and YOLOed in.

It's still just "buy low, sell high". You make the money from the same folks you normally would, only the order of when you buy and sell is swapped.

From people who bought the stock when you started to short.

The buyer of your short sale would lose money to you. Remember options are contracts between two parties.

Shorters are selling to willing buyers at the current fair market price. So that they may survive.

The market just redistributes wealth from less informed players to more sophisticated/informed ones.

1) borrow the stock

2) sell it

3) rebuy it at the lower price (assuming you're right)

4) give it back to whomever you borrowed it from plus a consideration for letting you hold what's theirs for a bit

Whatever's left after you return the stock and pay the interest is your profit, which comes from the people who bought it from you in step 2. If you're wrong, and the price goes up, you have to replace the stock you borrowed at a higher price than you got for it and that's your loss (which could potentially be infinite, as opposed to long positions where you can only lose what you initially invested)

The person you sold it to after borrowing it, who paid for it at the elevated price.

When you short a stock, you borrow shares from someone who is holding that stock and their broker gets money for lending the shares and sometimes the holder of the shares lent out gets money. You sell the shares, probably to a market maker. The cash is credited to your account and held as collateral.

Sometime later, the stock has fallen and you decide to close the position. You buy back the shares with the borrowed money probably from a market maker and close your position. You give the shares you borrowed back to the lender. Your net profit is sell_price - buy_price - borrow_fees, anything left is your profit.

Stocks are not zero sum like options or futures, they also have no expiration date (unlike derivatives), it’s possible a short seller sold shares to someone who later profited, and then it’s also possible to buy the shares from someone who profited, even if you made a profit on shorting the stock.

So the answer is “other market participants” who also may have profited on their buy or sell.

you borrow shares from a permabull and immediately sell them to whatever is buying

all you owe is the number of shares you sold, the original owner doesnt care what happened as long as they get identical ones back eventually. In the meantime, you pay interest on the initial value of what you borrowed and sold

You just sit on the cash

later when the shares are cheaper, you buy shares on the open market and give them back to the person you borrowed from

whatever cash is leftover from rebuying is your profit

Also, highly related with significant discussion in the past 2 days:

https://news.ycombinator.com/item?id=48933344 - "SpaceX stock erases all its gains and slides below IPO price in intraday trading" - latimes.com | 306 points | 1 day ago | 281 comments

https://news.ycombinator.com/item?id=48920181 - "SpaceX bond worth 10% less than issue price – heading for junk bond status" - ft.com | 561 points | 2 days ago | 603 comments