That’s correct, this is in relation to the Offensive Weapons Act and for blades articles age verification (at point of order and on delivery) is a requirement.
eBay have gone further than the requirements and made age verification a requirement for almost all blades articles.
age of eBay account isn’t one of the checks deemed appropriate by law (doesn’t display due diligence) so the only means eBay accept as proof of age is logging evidence of a credit card in your name.