I feel a bit strange about it, not sure why. They say they want to work out distance but I have given them town and first three digits of postcode. I am sure they can work out approx distance from there, but they have come back and demanded full postcode?
I can't see the scam but also can't see why they need it?
Ask the bidder of they feel comfortable giving you their postcode and you'll calculate the distance for them. Just remember to give them both the toad distance and the as the crow flies distance, just in case they want to argue the point if they win.