So, if you have a main street shop, it would be ok to have detectives monitor people as they enter the store (EDIT to add: or, actually, as they're window shopping), take notes as to what they buy, surreptitiously check their ID, and then follow them home (or place a bug on their car) to find out where they live, what car they drive, etc., so that you can better understand "who my customer is and why they're not buying my product"?