I think i may need to do some research about that part... check out some controversial documentaries and see how others have gone about it.

I have no problem videotaping people without their consent, but i would afterward request their permission to show them in my final video

I may also take some filler footage of the salespeople reacting to real customers with their knowledge, i would go about this by telling them ahead of time that i am filming for a project unrelated to their behavior so they do not realize what i am necessarily focusing on.