A Nasdaq-listed stock currently shows an inside market of 15.50 - 15.75, 10 x 10. A broker-dealer that is not a market maker in the stock simultaneously receives orders from two customers, one to buy 500 shares of the stock at the market, the other to sell 500 shares at the market. Rather than send the orders to a market maker for execution, the broker-dealer matches the two orders with one another at 15.625, and charges each client a commission. This transaction is: