Did you know a fiduciary relationship is forever? When working for a Seller or Buyer as a "Single Agent" you are creating a fiduciary relationship.
Brief definition from Wikipedia: for full definition click
A fiduciary duty is the highest standard of care imposed at either equity or law. A fiduciary is expected to be extremely loyal to the person they owe the duty (the "principal"): they must not put their personal interests before the duty, and must not profit from their position as a fiduciary, unless the principal consents. The fiduciary relationship is highlighted by good faith, loyalty and trust.
Once this relationship is established it does not go away just because your listing has expired or your Buyer is no longer working with you. You are still bound by this relationship. This means that whatever your client has told you in confidence must remain confidential until your client has given you permission to remove the confidentiality. And you have the duty of full disclosure to your principal. This is where in my opinion the problem comes in with having an Agency relationship in Real Estate.
Example: Suppose you have a listing as a single Agent and the Seller, your principal, has told you that she has been having financial difficulties. She is a few months behind on her mortgage payments and needs to sell quickly to avoid foreclosure. And she has asked you not to disclose this information to anyone. Ok, no problem, so far. A few months go by and your listing agreement expires and the Seller decides to list with another Company. About a month later you are searching for homes on the MLS to show the new Buyer that you are working with. This Buyer has signed a "Buyer's Broker Agreement" agreeing that you will work for him as a single Agent. The Buyer is now your Principal. Lo and behold the best match for your Buyer is the property that you had listed that is now with another Company. You take the Buyer to see it and he falls in love with the property. You rush back to your office and start writing a purchase offer and your Buyer looks at you and says "It looks like this property has been on the market for several months. Do you know why the Seller is selling?"
Here's your dilemma. You can't mention the pre-foreclosure or you are breaking your fiduciary relationship with the Seller and you can't not tell the Buyer because you owe him full disclosure.
This is a very simple and common example. It doesn't even take into account dual Agency or the fact that an Agency relationship starts with your Broker and filters down to all the Agents under that license. So what do you do?