It's the law, to prove you are over the age of 18, to buy any sort of knife in the UK, so the only way to prove this in online buying is to have a credit card on file, as anyone of any age, can hold a debit card.
So on your eBay homepage go to Account tab >> Personal Information >> Financial Information, and add your card there.
Add to Paypal too, click on the cog like icon top right, of your PP page, and you will see where to add card details on the left.
