A NYT article that shows a federal law actually exists to prevent people like Seung-Hui Cho from buying guns. A Virginia law is less stringent, but apparently the gun shop owner didn't know to check for court-certified mental illness based on the federal law. Theoretically, gun shop owners should have done their constitutional duty and went by the federal law.
In a state like Virginia, home to the NRA's gun museum (careful, if you're prone to seizures I would think twice about clicking on this link), the proud gun ownership attitude in this case has lead to lax laws. I can imagine the grumbles from gun enthusiasts now: "Grr. One person does something wrong and the rest of us have to suffer."
I understand. I'm from a rural area that has a lot of gun pride. But one point the NRA says (but has trouble backing up) that is valid is they want to promote responsible gun ownership. That includes background checks for mental illness and history of violent crime, showing new gun owners how to safely use a gun to prevent accidents, and encouraging gun owners to keep their guns in a locked cabinet so young children can't get to them.
Until the second amendment gets repealed (unlikely) anti-gun activists need to walk a fine line of strict regulation and enforcement. The good thing about the Virginia Tech tragedy is that it may put gun shop owners in their place about taking responsibility to thoroughly check buyers.