It's not eBay's law it's thr law of the land that you have to be 18 years old or older to be able to buy any knife anywhere in the UK.
The only way to prove this for online selling / on eBay is through owning a credit card as anyone of any age can own a debit card.
If you do not wish to own one, then perhaps ask a friend or family member who does have one, to buy this item for you.
@michrr2012