Can we agree that it is illegal even for private sellers to sell handguns to anybody who is under 21 years of age or not a legal resident of Colorado?

So how can you be certain that the transaction is legal without checking a pictured ID? I am not trying to make this a debate of the constitutional right. Rather, I am trying to find a balance between buyer's privacy and seller's legal cover.

I do recall that the two persons who sold the pistol to Klebold and Harris were convicted in the criminal case.