From The Federalist:

By way of background, firearm dealers are required to run a background check on anyone to whom they sell guns. “Universal” checks would impose the same requirement on everyone else.
Whenever there is a multiple-victim murder with a gun, Democrats don’t wait for law enforcement agencies to determine how the perpetrator acquired the gun. They immediately demand “universal” checks, to trick the public into thinking that the perpetrator could not pass a background check, therefore bought the gun from someone who is not a dealer, therefore the crime would have been prevented if the seller had been required to conduct a check.