"You will need to show a valid ukaraa and id to comply with uk regulations"
Unless you are a retailer, you have no right at all to ask for this information. You are not complying with UK regulation. Anyone can buy, own and sell airsoft replicas over the age of 18. And you don't even need to be 18 to own one.
So you might be ok to ask for ID (sounds like a data breech waiting to happen) but you do not need a Ukara to sell a replica second hand.