Here's one of my responses in an older thread. I've basically done the research for you.
NB: "deactivated firearm" by the definition in the document covers paintball, airsoft, and pellet guns (.177 cal and 4.5mm).
Restriction re sale of deactivated firearm
(2) No person shall sell or transfer a deactivated 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. 3 (2).
(That's just the search box since the results are based on "sessions" so the results are unique to each person/search).
NB: Elsewhere there isn't technically any such rule however most stores in other provinces generally do follow those kinds of established rules/procedures.