You cannot on eBay, they only accept a credit card on your eBay account which proves you are 18 years old or older, as anyone of any age can own a debit card.
If you wish to buy an item which required you to be 18 years + then perhaps ask a family member or friend, who does have a credit card on their eBay account, to buy this for you.
@sjrowlandstitanium36
