What professions give you a knee-jerk reaction to think 'scum bag' or scammer?
For me, I always had this kind of knee-jerk reaction to taxi drivers, used car salesmen, and, mostly in Bulgaria, real estate agents. In more recent times, I would add anything crypto-related to the list, too.
Car salesmen are probably the worst still I think. My uncle was importing and selling Japanese cars many moons ago, and the car salesmen he had on the lot were basically just criminals, constantly lying, stealing, and being shady af.

kidnapping rescuefew changes.ratsvery common at large events where parking lots are full and they may place wooden boxes on the streets, reserving spaces that would otherwise be free, but which they themselves invented and charge rent on the spot. I never pay them, even knowing the risk of them damaging or denting my car.