Straight out of the NRA playbook - why have a law that is difficult to enforce.
Bottom line: the NRA and paranoid gun lovers want to prevent law enforcement from doing their job - when a gun is used in a crime, law enforcement traces the serial number on the gun to the mfg who has a record of merchants they sold it to...If gun merchants do not use a background check system, law enforcement cannot do their job - thank you NRA and paranoid gun lovers.
"...the empty desks of 20 children marked absent today, and forever"
-Poet Richard Blanco at President Obama's Inauguration, 01/21/13