How Your Stock Trades Really Get Filled

The Corner Store and the Supermarket

Imagine you want to buy a bag of rice. You could walk into a big, noisy supermarket where prices are posted on the shelf for everyone to see. Or you could duck into a tiny corner store that quietly hands you the bag at a slightly better price. Both get you fed. But the two shops work very differently — and lately, more and more rice is moving through the corner store.

That's roughly what's happening in the US stock market. Nearly half of all US equity trading now happens off-exchange, meaning trades are executed directly by wholesalers and institutions rather than through a public exchange like the NYSE or Nasdaq. In November 2024, the US market crossed a milestone: for the first month ever, more volume executed off-exchange than on. The share stayed above 50% in December 2024 and January 2025.

Wait — What's a Wholesaler?

In plain terms, a wholesaler is a large trading firm that stands ready to buy or sell stocks directly from retail brokers. When you tap "buy" on your brokerage app, your order often doesn't go to an exchange at all. It's routed to a wholesaler, who fills it — sometimes at a price slightly better than the best publicly displayed quote.

This practice is called internalization. The wholesaler takes the other side of your trade from its own inventory. Your brokerage gets a small payment for sending the order its way — that's payment for order flow — and you get a fast fill, often with a cent or two of price improvement.

That sounds like a decent deal, and for many retail investors it is. Off-exchange trading frequently provides better prices than exchanges could have offered for the most commonly traded stocks, according to research from the Office of Financial Research. So why is anyone worried?

The Hidden Cost of Hidden Liquidity

Because the market is a connected ecosystem, what happens in one corner affects the others. The OFR research found that around an off-exchange trade, hidden odd-lot orders on exchanges briefly multiply but lose relative value — and the most valuable ones tend to get cancelled right before a trade. In other words, some liquidity shows up just long enough to be counted, then vanishes like a party guest who waves from the doorway and leaves.

These shifts happen in milliseconds. They typically reverse within half a second. But the pattern suggests off-exchange trading does briefly shape what's available on public exchanges — even if retail investors still seem to get good prices.

Who Really Pays for Your Free Trade?

A 2025 study in the Journal of Financial and Quantitative Analysis offers one of the most striking findings in this area. Researchers Barardehi, Bernhardt, Da, and Warachka looked at imbalances in internalized retail trades and found that wholesalers appear to use retail order flow to provide liquidity to institutional investors — especially when liquidity is scarce.

Think of it this way: retail investors and big institutions almost never trade directly with each other. Retail orders flow to wholesalers; institutions hunt for large blocks of shares elsewhere. The wholesaler sits in the middle, effectively connecting the two sides — and earning a spread for the trouble.

The researchers built a measure of institutional liquidity cost based on average absolute retail trade imbalances. Stocks with higher values on this measure were associated with larger expected institutional price impacts and annualized liquidity premia of 2.7% to 3.2% after 2010, even after excluding microcap stocks.

That's not a guaranteed return — it's a statistical pattern in historical data. And it raises a fair question for anyone holding a diversified fund: if your fund manager has to pay more to trade, that cost eventually comes out of returns.

Not Just Meme Stocks

You might assume this is mostly about small, wild stocks that day traders love. It isn't. Off-exchange trading is over 45% across all market caps, all price points, and exchange-traded funds. Within the Russell 3000 index, very few stocks now have off-exchange share below 30%, and most sit between 40% and 50%.

Dark pools — alternative trading systems that match orders away from public view — have actually stayed fairly stable as a share of volume. The growth has come from bilateral trades: wholesalers filling spread-crossing orders, often with price improvement and negotiated trades. For mutual fund managers trying to buy or sell in size, those bilateral trades represent liquidity they typically can't touch.

Why It Matters Beyond Trading

Public exchanges do one thing exceptionally well: they show everyone the same price at the same time. That shared reference — the National Best Bid and Offer, or NBBO — is what keeps markets fair and comparable. It's stitched together by the Consolidated Tape System, which processes quotes and trades from every venue into one feed.

When more trading migrates off-exchange, that shared picture gets blurrier. Research suggests there may be a tipping point — somewhere between 10% and 46.7% dark trading — above which market quality and spreads get worse. The US market has now crossed all those thresholds. Nobody knows for certain where the real tipping point is, or whether we've passed it. That's genuine uncertainty, not a scare tactic.

There's also a broader economic angle. Companies with wider spreads tend to face higher costs of capital, which can reduce investment back into the economy. So the plumbing of trading isn't just a trader's concern — it touches how businesses fund themselves.

What's Being Done About It

Regulators have been modernizing the disclosure rules that govern this space. Rule 605, adopted in 2000 and unchanged for decades, was amended by the SEC in March 2024. The updated rule expands which firms must report execution quality, adds new statistical measures, and requires a human-readable summary report. The compliance date is around late 2025 or early 2026.

The SEC itself acknowledged that the old Rule 605 reports were too complicated and hard to understand for the average investor. The changes aim to fix that.

Meanwhile, Rule 606 requires brokers to disclose where they route customer orders — quarterly reports broken down by month. That's the disclosure that lets you see, at least roughly, where your trades end up.

Importantly, these rules and their timelines vary by jurisdiction and are still evolving. What applies in the US won't necessarily apply elsewhere. And disclosure reports, as brokerages themselves note, aren't audited and may contain errors — they're a starting point, not a verdict.

What This Means for You

You don't need to panic about your brokerage app. For most retail investors, off-exchange execution delivers fast fills, no commissions, and often a fractionally better price. That's a real benefit, not a trick.

But it helps to know the trade-offs. Your "free" trade isn't free — it's paid for through the spread and through the value of your order flow. That's not a scandal; it's the business model. The question is whether the market as a whole stays healthy as more volume moves behind the curtain.

If you want to peek behind that curtain, ask your broker for its Rule 606 report. It's free, and they're required to give it to you. Think of it as reading the label on the rice bag — not because the rice is bad, but because knowing where it came from is part of being a thoughtful buyer.