It has been mentioned before but always good to remind people about it. As you say if people post a link to a car after searching within xx miles of a postcode, it keeps the post code in the link. i.e.
where you can see the SW1A 1AA postcode - the London residence of a certain well known lady.
That link was from clicking on a specific search result to then show the car, in this case a Signum 1.9CDTi. To post up the link to the specific car without showing the postcode simply requires all the code after the long number to be removed...
...and that shows the car without any reference to a post code.