Yep, those are eBay's rules, they require a credit card on your eBay account, if you wish to buy any ' bladed item', this the only proof they accept that you are over the age of 18. Anyone of any age can own a debit card.
It also has another purpose. With no credit card on your account, this also shows hopefully , your responsibility for anyone in your household who may not be of 18 years + , to not be able to use your account to buy bladed items.
It not only covers yourself , but members of your household with access to your account.
@grahamfrances