Ivory isn't like platinum. There are tons of recyclers that actually sell the precious metals they have obtained in a legal way. Scrap yards can sell their converters to middle men, who then resell them to the refineries. That makes it super hard to differentiate between legal and illegal bulk resellers for the refinery. In this case, the middle men (dgauto) handled the first recycling steps (removing the metal core from the converters), thus making it even harder or impossible to trace the metals back to stolen parts.

Ivory, on the other hand, can be assumed to be illegally produced in 99% of the cases. If there are legal ways to even obtain new ivory, it's on the buyer to prove that their source is legal. That's not the case for catalytic cores.