Sure the cops the, Sheriffs, the BATF, the FBI all share responsibility, but a quick read of the statutes in the link below shows the real cause of the problem.
That and the fact that too many nongs over there think it's still 1776 and the Gummint's out to get them, or brainwash them with vaccines. One specimen I was reading the other day insists that the World Health Organisation is actually a satanist conspiracy to enslave us all and that UNESCO is a conspiracy to breed all the races together and "muddy up" the bloodlines to get rid of the white man. No prizes for guessing there's a white robe and pointy hood in his cupboard.
According to the American Findlaw website, 28 of the 50 US states do not require checks, ID or licence for private citizens to sell guns to each other. All of those 28 have provisos, but they nearly all contain the get out clause that the seller can not "knowingly" sell to a prohibited person.
The only proviso for New Mexico gave me a giggle:
N.M. Stat. Ann. § 30-22-12(A): May not transfer firearms to a person in custody or confinement without the consent of the officer in charge.

http://consumer.findlaw.com/consumer-tr ... state.html