Many folks who sell stuff on Craigslist worry that they are inviting murderers into their house to look at their blender, or lamp, or sofa, or whatever. Imagine if the government said: here's a website that will tell you, instantly, if your potential houseguest is a violent criminal. A lot of folks would ask for names and DL numbers of Craigslist strangers, don't you think?
But now imagine that CL ad is for a gun. You'd want to be extra sure you weren't letting a strange murderer into your living room, right?