I've installed lots of covert cameras. In my experience, the three main customer profiles are:
1) HR or Security trying to get evidence before they fire or charge an employee
2) small business owners convinced their employees are trying to screw them
3) creeps
In the first instance, someone knows, or think they know, what's going on, but they need CYA proof. Internal theft cases are probably the biggest driver of covert video sales. Typically, you'll install a camera in a designated spot, the Security guy will leave some bait in plain view of the camera, the employee goes for it, and the video either gets turned over to the police or HR uses it as blackmail to get the employee to leave without making a scene.
The second kind of customer thinks he's the first kind of customer, but isn't. Whatever, as long as the check clears it's not your problem.
The third kind of customer is far trickier, and requires a lot of diplomacy to properly handle. The best analogy I can think of is gun sales, and there's lots of information on the internet on when, why, and how you can refuse to sell someone a gun. Basically, if someone feels "off", insisting on a credit or debit card and insisting on generating a bill of sale is usually enough to make the creepy weirdos just decide to buy the camera from Amazon and do it themselves.