If a subscriber wants to buy a stock through an ECN with no sell order, 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.

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The Answer of If a subscriber wants to buy a stock through an ECN with no sell order, 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.

If a Subscriber Wants to Buy a Stock Through an ECN