The government doesn't just ask you to file your taxes. They can investigate what you own and ask you to prove how you got it. They can use prosecutors and police to conduct searches and investigations. The police can also inform the bureaucrat that they suspect you're braking tax laws. And yes, if the police see someone carrying a bunch of catalytic converters they can stop them and ask to prove how they got them. You're talking about it like police investigating ownership is something new and dangerous while in reality it's pretty standard thing. They will investigate all sort of suspected activity and carrying large amounts of cash or often stolen items can be one of them. This "violent arm of the government" interacts with people all the time, pretending they are some kill squads that should not get close to people unless they have evidence of crime or something is silly.