Final answer:
Engaging in private securities transactions without written prior consent of the employing broker-dealer violates FINRA rules regarding selling away.
Step-by-step explanation:
The action that violates FINRA rules regarding selling away is engaging in private securities transactions without written prior consent of the employing BD. Selling away refers to when a broker performs transactions outside the purview of the brokerage firm they are affiliated with, without notification or approval. FINRA rules mandate that registered representatives must provide written notice to their employing broker-dealer (BD) and obtain written prior consent for any private securities transactions in which they participate. This rule ensures oversight and the prevention of conflicts of interest or fraudulent activities.