The only way to mandate a background check for a private sale is to know that you are selling. The only way to know you are selling is by knowing that you have one. The only way to prove your gun was purchased via background check would be through some sort of licensing. So your choice becomes gun registration or become a criminal. This isn't a slippery slope. It's an icey cliff. For my intellectual friends, of course I've oversimplified this. But a scenario either anchored in, or loosely based on my scenario are the likely paths.