If a subscriber wants to buy a stock through an ECN, if there are no sell orders, the order will be executed and then matched after a sell order arrives.
Absent a sell order, the transaction cannot be executed. It can, however, wait for a matching sell order, or can sometimes be rerouted to another exchange.

Question

Quizplus · Accepted Answer

A buy order cannot be executed without a matching sell order. It will remain pending until a sell order is available or may be rerouted to another exchange.

If a Subscriber Wants to Buy a Stock Through an ECN