Make it required by law, and they won't be able to go somewhere else. Legitimate businesses with scrap to sell aren't going to risk committing a felony to get paid below market value 30 days sooner (Not to mention that many businesses are used to getting paid on a delay, and selling scrap for pennies is a tiny portion of their revenue flow).
Catalytic converter thefts have been decimated by similar policies. As it turns out, there are plenty of hoops that you can introduce that aren't a serious impediment to b2b transactions (which make up the overwhelming, massive majority of legitimate trade in this space), but are a serious impediment to tweakers selling stolen property.
> Or the drug dealers take the copper as payment and then don't care about the 30 days if it means they can sell more drugs
Street dealers are trying to run a drug business, not a goddamn copper clearinghouse. If they wanted to run one, they wouldn't be out on the street slinging meth.