How would they know if what was happening is legal or not without following up on the "reports" ?
Because nothing in the reports of a man selling firearms at a yard sale would suggest that anything illegal is happening.
Would you expect the police to dispatch an officer if passers-by called 911 and reported that he was selling baseball bats, hunting knives, axes, or any number of other completely legal (though potentially dangerous) items? No, of course not.
What many of us are pointing out is that possessing a firearm, and selling a firearm are not AT ALL indicative of unlawful activities, and it is inappropriate for law enforcement to treat a citizen as though he needs to be investigated for engaging in activities which do not suggest or imply any law breaking. It smacks of harassment and intrusiveness.
Citizens don't need to be checked up on to make sure they aren't law-breakers. They need to be left alone until/unless some evidence is presented that a law is being broken.
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Several very good analogies apply:
1) "Sir, I see you with a very nice watch there. I'm going to need to see some identification and your receipts to prove you didn't steal that. Sure it COULD be yours, but I'm going to need to see proof."
2) "Ma'am, I've observed you walking out of the mall with these two children. Please step over to the car here and produce your identification and birth certificates for those kids. While having kids with you in public is perfectly legal, we have to check that you didn't just kidnap them..."
3) "Sir, you were observed leaving X,Y,Z firing range. Open your trunk, I'm going to inspect your firearms and run the serial numbers to make sure they aren't stolen..."
4) "Sir, you purchased cough medicine from CVS drug store a few moments ago. While you probably do have a cold, I'm going to need to inspect your apartment to make sure you aren't making methamphetamine."
5) "Sir, someone called and said you are selling guns at a yard sale, which is perfectly legal in this state, but I need to walk through your house and look into your gun safe to satisfy myself that things are on the up and up here..."