Hi!
I’ve done shops before security, in all of my airports. In Sydney International my local airport, we have shops in the Arrival section of the Terminal. So I use this as inspiration for putting shops outside of security.
As for amount of people that pass through, it depends where you place it. I usually place it in the area where passengers first enter / leave the terminal, near the escalators to the Metro.
When I do this, it does much better then even the shops inside the security area, given you are handling both incoming and outgoing passengers.
Hope this helps!