[Mr.Shiney]

The problem is you have you prove that the retailer has knowledge that the buyer of the airsoft gun is intentionally purchasing it as a "straw buyer". If the retailer knows that he is selling the airsoft gun to someone, that intends to pass it on to a minor, that would be an offence.

RSO Imitation Firearms Act.

But there is the wrinkle, you have to prove that the retailer knows this. Otherwise, just bad judgement.

Restriction re sale of imitation firearms

4. (1) No person shall, in the course of running a business, sell or transfer an imitation firearm to an individual unless the individual is at least 18 years of age and presents valid identification in accordance with section 5. 2000, c. 37, s. 4 (1).
Reliance on documentation

(3) A person who sells or transfers an imitation firearm to an individual on the basis of the documentation described in section 5 does not contravene subsection (1) if there is no apparent reason to doubt the authenticity of the documentation or that it was issued to the individual providing it. 2000, c. 37, s. 4 (3).