It's in place for all sellers and buyers, that is, a verification that one is over the age of 18 to buy any bladed item. Not eBay's rules, but the rules in the UK.
eBay will only accept a credit card as verification because those Companies have already gone through a stringent process to verify that person's age, before granting them their card.
They ask for a card again when buying, because this is to stop another person perhaps underage, from using your account to buy a bladed item, it's a good security measure.
@prochet32-uk