I have mixed feeling about gun control laws and I don't think that ban on military guns are effective to prevent the tragic events.
The gun dealers could refuse the sale if they know that you are homicidal or motivated to kill people.
Yes, but outside of the gun dealer in Missouri refusing to sell a gun to someone who "didn't look right" how often does that happen. Banning military guns from private sale won't prevent tragic events from happening, but it will cut down the number of victims. You aren't going to keep someone from acting on their intentions but you will cut down the damage and death they can inflict.