There's a reason universal background checks haven't already been enacted: The federal government lacks the authority to enact it.
Just as the federal government can't tell you how fast you can go on a local roadway, it also lacks the constitutional authority to regulate a private, non-interstate transaction between you and your neighbor for privately owned goods that are legal to own.
There's no federal issue in a private, face-to-face gun sale of a used firearm between to residents of the same state. States can regulate that transaction, and some do. Others don't.
If Americans don't push back against a universal background check law, we will have allowed the federal government to once again overstep it's legal role.
P.S. I'm fine if states want to do it. That's not unconstitutional.