Internalisation And Wholesale Market Makers

When a retail investor clicks "buy" in a brokerage app, the order usually doesn't go straight to a public exchange. Most brokers route it instead to a small number of large, specialized firms called wholesale market makers, who fill the order themselves out of their own inventory. This is called internalisation — the trade never touches the exchange's public order book at all, even though the final price is required to reference it.

Why this arrangement exists

A wholesaler makes money on the spread between what it pays to fill a retail buy order and what the same instrument is trading for on public markets, at a volume large enough — aggregating millions of small retail orders across many brokers — for even a fraction of a cent per share to add up. In exchange for this flow, wholesalers typically pay brokers for the right to see and fill it, an arrangement known as payment for order flow, and in return promise the retail client a fill at least as good as the best publicly available price at that moment, often slightly better.

Worked example: filling a retail order internally

A retail investor sends an order to buy 100 shares of a stock currently quoted on public exchanges at $50.00 bid / $50.02 offer. Instead of routing to an exchange and paying $50.02, the wholesaler that receives the order internalises it, filling the client at $50.015 — half a cent better than the public offer, satisfying the requirement to give the client "price improvement." The wholesaler itself effectively bought the shares (from its own inventory or by trading elsewhere) closer to $50.00, pocketing roughly a cent and a half per share on the trade, a profit made possible because retail flow, in aggregate, tends to be far less informed than the flow arriving directly on exchange order books.

What this means in practice

Internalisation is controversial precisely because the incentives aren't perfectly aligned: a wholesaler paying for order flow is buying access to flow it expects to be profitable, and critics argue this creates pressure to route based on payment rather than pure execution quality, even under a "best execution" requirement. Defenders point to the price improvement metric above — retail clients frequently do get a better price than the public quote, something that wouldn't happen if wholesalers were simply skimming from uninformed flow without competition. Regulators require public reporting of execution quality and payment-for-order-flow arrangements precisely so this trade-off can be measured rather than taken on faith.