It provides the perfect blend of anonymity and transparency, giving other users reassurance that they are dealing with other legitimate licence holders. Your firearms licence number on its own does not identify you to anyone and under the privacy laws the Police are not able to disclose any personal information about you, except to confirm whether or not your firearms licence number is valid including the endorsements you claim to hold. We are simply trying to add some transparency and traceability to every sale - ultimately this is for your own protection and we would discourage you from using our site if you feel you have something to hide.
NOTE: This extra precautionary measure does not preclude you from having to complete all the usual Police paperwork or physically sighting the persons licence if you are dealing face to face!