How is there always a buyer?

You can sell shares in seconds at a price you saw in advance. Nobody found you a buyer — a market maker quoted both sides and earns the spread. Liquidity is a manufactured service, and it thins exactly when you need it.

Part of how markets actually work.

Here is something that should be more startling than it is.

You decide to sell 100 shares of a large company. You press a button. Seconds later they are sold, at a price you could see before you pressed it. You did not find a buyer. You did not negotiate, or wait, or accept a discount for wanting out today.

Try that with a house.

So who bought your shares? Nobody woke up that morning wanting exactly 100 shares at exactly that moment. And yet the trade happened instantly, and would have happened just as fast at three in the afternoon on a Tuesday for a stock you have never heard of.

The answer is a piece of machinery most people never have explained to them, and understanding it changes how you read every price you see.

The market maker

Your shares were almost certainly bought by a market maker: a firm whose entire business is quoting, continuously, two prices on thousands of securities at once.

• A bid — the price at which it will buy from you
• An ask — the price at which it will sell to you

The ask is always higher than the bid. The gap is the spread, and it is the firm’s revenue.

The market maker has no opinion about the company. It does not want your shares and will likely be rid of them within seconds. What it wants is to do this several million times a day, capturing a fraction of a penny each time, while never accumulating a position large enough to hurt.

That is the trade: it provides certainty of execution, and is paid a small amount for it, very often.

What you are actually paying

This reframes something that trips up a lot of people.

You buy at the ask and sell at the bid. The moment you buy, you are down by the spread — not because the price moved, but because you crossed from one side of the quote to the other. To break even, the price must move in your favour by at least the spread.

On a heavily traded stock the spread might be a penny on a fifty-pound share, which is nothing. On a thinly traded small company it might be several percent, which is a very great deal — and it is the same trade, executed the same way, feeling identical when you press the button.

This is why “commission-free” is a precise and slightly misleading phrase. There is no commission. There is still a cost, and it is in the spread, and it does not appear on your statement.

Many brokers are paid by routing your orders to trading firms rather than charging you — payment for order flow. That arrangement is legal, disclosed in the small print, and worth understanding for what it implies: if you are not paying the broker, the broker’s customer is the firm on the other side of your trade.

Where it breaks

The critical property, and the reason this matters beyond curiosity:

Market makers are not obliged to be there.

They quote because it is profitable, and it is profitable when conditions are normal — when they can hedge their inventory and estimate risk. When the market moves violently, both of those get harder. So they widen their spreads and reduce their size, or briefly step away.

Which means liquidity is thinnest exactly when the most people want to sell. The guarantee you thought you had is strongest when you do not need it and weakest when you do.

This is not a defect. It is what the arrangement always was: a commercial service, priced by risk, provided by firms with no obligation to lose money on your behalf.

Anything you hold on the assumption that you can exit instantly should be sized as though that assumption fails on the day it matters. In extreme conditions it has, repeatedly.

The order book

Underneath the quote sits the order book — a live list of every unexecuted order: who wants to buy how much at what price, and who wants to sell.

Two kinds of order:

• A limit order says “I will trade at this price or better.” It sits in the book, waiting, adding to the depth available to others. You are supplying liquidity.
• A market order says “I will take whatever price is available now.” It executes immediately against what is already there. You are consuming liquidity.

Market orders are why the button feels magic and why the price is not always quite what you expected. If your order is larger than what is available at the best price, it eats through the book — filling part at one price, part at the next, and worse as it goes. On a small trade this is invisible. On a large one in a thin stock it is the dominant cost.

What this means for you

Not trading advice. Structural facts with consequences.

Liquidity is a feature of the security, not of investing. Large companies and broad funds are liquid. Small, exotic and thinly traded things are not, and the difference costs real money on the way in and considerably more on the way out.

The spread is a cost of activity. It is charged per transaction, so it scales with how often you trade. Someone trading weekly pays it fifty times a year; someone buying and holding pays it twice a decade. This is one of the concrete mechanisms behind the finding that frequent traders underperform — the cost is structural, not a matter of skill.

Someone competent is always on the other side. Not sinister — market makers are indifferent to direction. But it does mean every trade you make is one someone else was willing to take. Being able to say why you are right and they are wrong is the minimum bar, and “I read an article” does not clear it.

The part worth appreciating

It is easy to read this as a warning. It is also worth sitting with how remarkable the arrangement is.

A person with a few hundred pounds can buy a fractional claim on one of the world’s largest businesses, at a fair public price, in seconds, and reverse it just as fast. That is genuinely extraordinary. It is the reason ordinary people can access the ownership channel at all, and it did not exist for most of history.

The machinery has a cost and a failure mode. It is still one of the more impressive things people have built, and the price of admission — a fraction of a percent, paid invisibly — is remarkably cheap for what it buys.

Just know that you are buying a service, that it has a price, and that the guarantee has conditions.

Next in this series: how the advertising auction works — a market that runs entirely in the time it takes a page to load.