[hawleyal]

> This implies that Citadel is buying the stock before selling it to you.

Large firms do this business as usual. They maintain inventory and sell it to their customers directly, while still reporting the trade to the market audit trail. This is usually done by the private brokerage side of the firm executing all trades through the public investment side, and the investment side just executes the trade against the house inventory instead of a third party buyer/seller in the market.

[gpderetta]

Of course they do, but if they don't get a guaranteed buy on the exchange at the current best price, they can't guarantee profit.