Flip, as the seller he has the absolute right to decide who he sells to. There is no compunction to sell to anyone. In this case, he has to weight the risk of prosecution under the VCRA. If you cannot satisfy him that you are a person who has a legitimate reason to possess an RIF, then he has no defense in law. He can absolutely set whatever level of proof he feels happy with.
UKARA is not the only way of proving a good reason, but it is the simplest for someone to check. If you say "I'm a member of XXX site, here's their number, ring them and ask them" and he is of a suspicious mindset he may well think "actually, what's to stop him giving me his mates number". You see the difficulties? If he checks with UKARA and gets a positive answer then he has shown due diligence in law, in ensuring that you have a good reason, and so he does not risk prosecution.
If it matters to you that much, it's not too difficult to get UKARA registration.