When it comes to stuff like this, it's probably best to have some scruples.
Here's an article that explains how this loophole works, citing specific examples of how it can go horribly wrong. And this post, in which the author conducted an abbreviated version of the same experiment, boils the problem down to this punchy analogy:
"Requiring background checks in some places but not others is like locking three of your car doors, leaving one unlocked, and expecting that car to be secure."
Universal background checks are sounding pretty reasonable about now, eh?