Good question! Yes, you do need to check that the person you are selling to has the required defence (for our purposes an airsofter or reenactor). Bit what exactly you require as proof is up to you since it is you who may be breaking the law not the purchaser). In other words, think about the scenario of you being arrested for flogging guns to someone you shouldn't have and taken to court - what would a magistrate accept as evidence that you took reasonable steps to verify the purchaser's bone fide?
Such things as a photocopy of the buyer's current airsoft club membership card, a photocopy of his reenactment association membership card, photographic evidence you have been playing airsoft with the chap for years, a letter from the chap's airsoft club, may be even a UKARA number (although you have no way of knowing this is kosha as far as I know UKARA don't very members except to their shops/dealers - but you might find someone who is part of the UKARA scheme who can do you a favour).
So, summary: take all the reasonable steps you can, keep paperwork, and hope you don't get a knock on the door three months or a year later ('cause it would be a real bugger to have to answer for yourself in court). On the flip side, I haven't yet heard of anyone being taken to court for selling a RIF to someone they shouldn't (apart from market stall traders and skanky shops) as the self-regulation within airsoft seems to be working just fine.
may be even a UKARA number (although you have no way of knowing this is kosha as far as I know UKARA don't very members except to their shops/dealers - but you might find someone who is part of the UKARA scheme who can do you a favour).
The UKARA number is associated with an address and postcode. Sending only to that address is a pretty good way of making sure it's legit. As for checking: some shops will do the check for you. I think Zero One will. I know of a Facebook-hosted UKARA member that definitely will check for free.
Certainly that might put your mind at rest but is that evidence you can put before a court - 'a friend told me'? And is it a breach of the Data Protection Act for UKARA to divulge database contents to third parties? UKARA is there to protect participating retailers, not all and sundry selling second hand RIFs. Anyhoo, if you can get that confirmation in writing then you are laughing.
But, as a general warning, just getting a UKARA number from a purchaser with no means of establishing bone fide ain't worth a ha'pence. Maybe UKARA are missing a trick here, there would be a good revenue stream for making third party access possible. In the meantime, I reckon photocopies of a site membership card with stamped attendance dates and a copy of something like a passport or drivers license (pretty much what is required to get a UKARA number anyway) should be sufficient documentary evidence. (But then it's not me in front of the beak - sellers need to take the steps they feel are sufficient).
I sent them an email, asking if X person and number was registered.
though come to think of it their responce was 'yes they are, but we don't understand why you would want to know this', I said it was for 'the selling guns etc' and they went 'meh, OK' (note I am paraphrasing), though it did feel like was in some way 'imposing' by asking - they verified it though.
Same. I sent the info to the UKARA member and asked them to confirm it was valid.
When I've done it I also contacted their site to get them to confirm they were a regular.