[onewhippedpuppy]

John and Jane Doe may have been working with a realtor at the time, and just didn't tell you. If so, they probably signed an agreement to not purchase a home without first going through the realtor. It's a bit slimy, as the realtor didn't even show their clients your home. Essentially you'd be paying the realtor a fee to broker the sale.

If you have the contact info for John and Jane, you could fight back by directly contacting them and offering to sell the house to them directly for a lower price. Of course, you still have to pay someone to broker the deal.

EDIT: tell the realtor that you don't feel responsible for paying the entire commission because they did not bring you the client, or show the home. Find out what a dedicated real estate broker charges (flat fee) then offer that amount to the realtor. Negotiate the home price seperately so John and Jane feel they're getting a better deal.