Quote:
"( 1 )Any person who sells to a person under the age of eighteen years an article to which this section applies shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months, or a fine not exceeding level 5 on the standard scale, or both.
(2)Subject to subsection (3) below, this section applies to—
(a)any knife, knife blade or razor blade,
(b)any axe, and
(c)any other article which has a blade or which is sharply pointed and which is made or adapted for use for causing injury to the person"
[Section 141a, Criminal Justice Act 1988 (as amended)]
https://www.legislation.gov.uk/ukpga/1988/33/section/141A
So it's the person who makes the sale who is guilty of an offence.
Eddie's assumption that "they looked older enough" would be a valid defence is incorrect. In order to avoid being convicted it would be necessary for the person who sold the knife "to prove that he took all reasonable precautions and exercised all due diligence to avoid the commission of the offence". In order to achieve that he would need to ask for proof of age (such as a passport or other form of age-related photographic ID) and to carefully check that the photograph matched that of the purchaser.