It depends. If you have created an agency relationship in which you’re representing the buyer—even if you haven’t signed a representation agreement—then you have a fiduciary duty to the buyer and you are not a sub agent of the seller.
A sub agency relationship is not as common as it used to be, but it usually arises when a buyer who is not represented uses the services of a broker to view a property. That broker then owes a fiduciary duty to the listing broker and the seller—not the buyer—and the buyer is treated as customer of the broker. It is important that a broker in this situation obtain permission from the listing broker, and explain his or her role to the buyer to avoid any confusion.