The profit motive is exactly why such regulations are useless. The dealer will harass every honest customer in case one of them are the type to report them for not checking, and then pay pennies on the dollar for the copper pipes ripped out of a building which they know were stolen because both parties to the transaction are aware they're profiting from participating in criminal activity.

And the dealer may not even need to be in on it. If the thief lies and says the material was found in a dumpster at a construction site, or that it has been collecting dust in their basement for twenty years, and there isn't any way to trace a generic pile of metal back to where it actually came from, what does it matter that you take their name?

Which is why cities like Detroit are full of buildings that have had their pipes stripped out. The deterrent isn't the scrap metal dealers, it's that the thief goes to jail if caught in the act, so it only happens in places where people reach a level of desperation that isn't present in most other places.

And regulating pawn shops is even sillier, because the thief could just sell the stolen goods on Craigslist or at a swap meet or a hundred other alternatives.

These types of criminals are most commonly caught directly by the victims -- because they're the ones with the best incentive to actually do the catching. The people with the second best incentive are law enforcement. Deputizing disinterested third parties, much less ones with an active conflict of interest, is largely useless and generally does more harm than good.