what do you mean "flippantly"? You don't need to be "such criminals" to purchase guns on the black market. You just need a connection and a reason.
People who knowingly break laws are being flippant to the government.
Again, gun enthusiasts imply that virtually all shootings are committed by people who don't follow laws and will just buy black market guns without regard to what's legal and what's not. I suspect that they greatly exaggerate the % of shootings (including accidents and suicides) that are committed by people with the necessary black market connections and necessary flippant attitude to do so.
And if I'm right, the whole argument of "gun control never works because criminals can get guns anyway!!!" doesn't really hold together logically.