Both seller and buyer have to have a valid credit card on their eBay account.
Be aware , not all bladed items / knives are allowed to be sold on eBay.
If you are selling tools for example, which come under this policy perhaps a polite message on those listings reminding buyers they have to have a credit card on their eBay account, as this is the only form of age verification eBay accept that the buyer is 18 years old or older.
@tonycharvil