I agree with your first point. The "black market" isn't a place you just go to if you want to buy illegal shit. The fact is, if the shooter didn't have legal, easy access to an assault rifle, he wouldn't have been able to do the damage he did. Period.
"It is not entirely coincidental that Aurora and Columbine happened 20 miles from each other."
Well, yeah, it is. Assault rifles are legal in most states. What does being in Colorado have to do with anything?