Many are faced with this dilemma, but rules are, and it's not eBay's rules but UK rules, that one has to be over the age of 18 to buy any bladed item.
I do understand you've had your account for 15 years so obviously didn't open it when you were 3/4 years of age, but an underage member of the family could use your account to buy, say any form of knife, hence the importance of having a credit card on file to prove your age.
A credit card is the only proof eBay accept as an online platform for proof of age, as anyone of any age can own a debit card.
You could ask someone who has a credit card on their eBay account to buy this item for you, that's what other members have suggested who've been faced with the same.
@richbell9999