Ask yourself what a reasonable person would do in your position?
You're selling a firearm to someone you don't know. What would you check at a minimum said person has to acquire said firearm.
(What happens after C-71 is a different story though).
A reasonable person would ask to see their license, for a face to face. It is also reasonable for you to assume the buyer has an authentic license.
If you can't see his/her license, you ask for the relevant details (name, full pal number and expiry) and call the CFC/CFO to confirm that those details are valid.