Let's look this shyte up before blathering on about things we don't know.
"The National Instant Criminal Background Check System (NICS) is a background check system in the United States created by the
Brady Handgun Violence Prevention Act (Brady Law) of 1993 to prevent firearm sales to people prohibited under the Act."
en.wikipedia.org
"Kid of legal age plops down his money and ID."- Here ya go.....looked it up for you. You ought to try searching online- all of the cool kids are doing it.
Minimum Age for Gun Sales and Transfers* Unlicensed Persons Handguns: Under federal law, unlicensed persons may not sell, deliver or otherwise transfer a handgun or handgun ammunition to any person the transferor knows or has reasonable cause to believe is under the age of 18, except under...
www.atf.gov
You're barking up the wrong tree WRT murders and gun sales- they're sold privately far more than through dealers.