You didn't read the earlier discussions on this subject. If those people have been diagnosed by a psychologists as a threat to society, then yes, they should not be allowed to buy guns. Does that mean they can't buy one later after dealing with there issues and being cleared by a psychologists? No.
Except that at gun shows in many states, the sales of arms can be accomplished without ANY background check whatsoever. IMO, one of the more sensible gun control laws would be to eliminate this glaring loophole; yet every time it is proposed the NRA and it's supporters fight it to the death and behave like it is the first step of a slippery slope in which all guns are seized by a dictatorship government. It is maddening that these people are so paranoid; there seems to be no reasoning with them.
You've been told this is a myth. Every law that applies outside gun shows applies inside the doors. Stop it.
Per Wiki

resently, 17 states regulate private firearm sales at gun shows. Seven states require background checks on all gun sales at gun shows (California, Rhode Island, Connecticut, Oregon, New York, Illinois and Colorado). Four states (Hawaii, Maryland, New Jersey, and Pennsylvania) require background checks on all handgun, but not long gun, purchasers at gun shows. Six states require individuals to obtain a permit to purchase handguns that involves a background check (Massachusetts, Michigan, North Carolina, Iowa, Nebraska). Certain counties in Florida require background checks on all private sales of handguns at gun shows.
The remaining 33 states do not restrict private, intrastate sales of firearms at gun shows in any manner.[16][17]