"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.
Have you considered a ZCI unit from @ak2m4?
Similar design, and a great unit.
https://www.ak2m4.co.uk/internal-parts/hop-up-chambers/zci-rotary-hopup-chamber-plastic-m4
A buyer needs a defence or the seller is committing an offence, new or or second hand. A UKARA registration is one such method though all it is is a centrally held copy of your site membership. The site membership is you're defence, the UKARA Database merely makes it easier to check. It is intended for Dealers and the Customs to use, though some dealers will check for you if you ask.