> Yet the thieves can still somehow trade the metals.

It isn't hard to melt down the metals and extract the valuable ones. A proper factory can do a much better job (get more of the valuable metals out), but it is something I could profitably do in my backyard over a fire. I haven't looked into the details of refining platinum, but I doubt is is much different from any other metal. (when I looked into it I lived in an area with a lot of iron deposits, I've since moved and no longer have access to iron ore so that potential hobby is dead to me)

Once you have raw metals you can claim any source you want. Just form them into a ring and say you were saving it for your anniversary, but your wife left you - I'm sure you can come up with other ways to make it look like got a legal source.

I don't understand the legal requirement thing. In my state, I'm pretty sure I could put a straight pipe down there and I'd never have an issue. In fact, I'd probably look into it if it happened to me. That said, I know my state is pretty lax about that sort of thing. Curious how it's actually enforced in others.

The law changed to prohibit the sale of used converters for installation or the installation of used on a different car, but I can still buy a new one and install it in my driveway if I want. (You might have a law in your state to prevent that, I guess.)

my neighbor had their's stolen in the parking lot of a grocery store while they were shopping.