The above link states that, even if a mall security guard conducts an unlawful search, the fruits of that unlawful search are admissible because that guard is not an agent of the police. Fine.
But, if the guard is not an agent of the police, and he is conducting an unlawful search, I have the right to physically resist that search. And, knowing that the fruits of the illegal search will be admissible in court, I just might have an incentive to resist to the extent that I seriously injure that guard.
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I'm not sure that signs would make a search legal. If I post a sign saying that anyone who comes into my store will be punched, that doesn't mean that I can legally punch a person.

