If you have copper of unknown origin you shouldn't be able to sell it, just recycle for free.
Copper has value. If people have this copper in their garage they can then get some value. The copper then goes back into the economy. Win Win. However if there is no gain to recycling it it might not quickly end up back into the economy. This is of course bad.
A totally radical view is to simply outlaw stealing :)
However are you saying that someone selling some extra scrap copper is in any way similar to someone selling their gun? These things are somehow equivalent?
However I will show you the actual connection. What you are doing is making the law abiding citizen be punish for the crimes of the criminal. This happens with fire-arms legislation as well.
The result tends to of course be that citizens lose rights while criminals do as they have before.
This is not a world I want to live in and I hope you would not want this either.