realtor,
If the other suggestions don't work, I'd certainly put this type of prospect on a contact list - descriptions of your marketing plans, success stories with past listings, information about the market in your area - for e-mail, mail, telephone contact.
When they find it's not so easy to sell their house themselves, you may find that the person they promised the listing to was a phantom anyway, and you'll be top of mind.
Hope that helps.
Kathleen