It's much more difficult to cloak, because the transactions are necessarily tied to a physical address. If you're okay with being identified, why not use the legal markets? If you're not okay with being identified, you probably don't want to leak a physical address.

I struggle to think of useful ways of cloaking the physical transaction. You can have it shipped somewhere you don't live, but then you have to get there to pick it up, probably leaving a massive trail of GPS data and gas receipts.

I wouldn't be surprised if shipments of gold or cash raise eyes at customs, either.