Breakdown
The Plumbing, Layer by Layer
Market makers, wholesalers, PFOF, and dark venues \u2014 what each is and what it does to your order.
01
What a Market Maker Actually Does
A market maker quotes a two-sided market: a bid where it will buy and an offer where it will sell, in size, continuously. If the spread is $10.00 bid and $10.02 offered, the firm buys from sellers at $10.00 and sells to buyers at $10.02, earning two cents per share for standing there and taking the other side.
It is a real service. Without someone always quoting, a seller has to wait for a buyer to arrive at the same moment — which is what illiquid markets feel like: wide spreads and violent moves. Market makers convert that into a continuous, tighter market.
The risk is inventory. A market maker filling buyers all morning is accumulating a short position, and if the stock runs, the spread it collected does not cover it. So market makers hedge constantly and want the flow they take on to be uninformed — which is exactly why retail flow is valuable to them and institutional flow is not.
02
Wholesalers, Internalization, and PFOF
A handful of wholesale market makers handle an enormous share of retail equity orders. Your broker routes your order to one of them instead of to an exchange; the wholesaler fills it from inventory. That is internalization. In exchange for the flow, the wholesaler pays the broker — that is payment for order flow, and it is a large part of how commission-free trading is funded.
The broker owes you best execution, and wholesalers typically fill at or slightly better than the public quote, which is the defense of the arrangement: you often get a fraction of a cent of improvement. The critique is that you were routed to whoever paid the most for you, that the improvement is smaller than the spread being captured, and that a meaningful share of volume prices off a quote it never interacted with.
Both things are true at once. For someone buying 100 shares and holding for a year, it is irrelevant. For someone taking hundreds of intraday round trips, execution quality is a line item on the P&L, and it is a legitimate reason active traders pay for direct-access routing where they choose the venue themselves.
03
Dark Pools and Where Size Hides
Dark pools are private venues where orders are not displayed before execution. They exist because showing a large institutional order on a public book moves the price against it before it fills. Executing quietly is a legitimate need, and prints report after the fact.
For an intraday trader the practical consequence is that the visible book is an incomplete picture of supply. A level that looks thin on level 2 can absorb far more than displayed because size is sitting off-book. This is why executions matter more than displays — time and sales tells you what really happened.
Displayed size gets pulled, refreshed, and shown to influence behavior. Treat the book as a hypothesis and the tape as evidence.
04
What This Means for How You Trade
First: market orders hand the other side the choice of price. In fast or wide-spread names, that is the most expensive habit in retail trading. Limit orders cost you some fills and save you far more in slippage.
Second: if execution quality is material to your strategy, route deliberately. That is the entire premise of direct-access brokerage — you select the venue and see the routing rather than being handed a fill from wherever paid your broker. I built businesses on that difference, and it matters most exactly where retail routing is weakest: illiquid names, wide spreads, and fast tape.
Third: stop reading the spread as a conspiracy. It is the price of instant liquidity. Your job is to decide whether you need to pay it right now, and to be the one setting the limit whenever you don't.