The law states that a buyer of a RIF must be over 18 and have a valid defence.
This is done by retailers with 'defences' such as UKARA, Reenactment Society membership, Museum Curator, Film/TV production.
For private sales its the sellers responsibility to ensure the buyer is a legitimate airsofter.
You can sell to anyone (if you think they are legit) BUT if that person then commits a crime with the RIF then it can come back to you being prosecuted depending on the gravity of the crime.
(Has there been a case of a seller being prosecuted?)