Buyers that call and ask to see property that you show them, then decides to use some other agent to write up an offer for them.  It does happen!  This may be a buyers market, but no excuse for such actions.  Let the agent know up front that you have or is working someone else.  No, they may not show you the listing, but at least you are honest.  Is'nt that what we should be looking for in each other, honesty and trust?