Unlike a trade initiated directly by the customer, an agency trade is when a broker-dealer, such as T.D. Ameritrade or Charles Schwab, initiates a trade on behalf of the client. The broker usually gets the wholesale (cheaper) brokerage price on the commission and terms. In other words, it's the agency's trade (and they have strong negotiating leverage), even if it is on behalf of the client's account (who has relatively weak negotiating leverage).