Earlier quoted context omitted.
Then how would we expect a legitimate mechanic to replace one?
Then they can show their mechanic's license. There's no reason for Joe off the street to come in with a backpack full of these things. Especially ones that are brand new. Here's 20 stolen from a dealership the other night - https://abc13.com/la-porte-car-dealership-theft-catalytic-co...
Scrapyards already need licenses (permits). Adding licensing requirements will just result in the people with licenses buying from the thiefs. It's just another middleman in the chain.
If anything it makes the problem worse because scrapyards often do turn away obvious thieves. A mechanic showing up with ten legit converters and forty stolen ones he bought off the thief gives the scrapyard plausible deniability. Furthermore it moves the illicit transaction outside of a legit commercial establishment and to wherever the thief and the mechanic care to meet.
It's already acknowledged that without enforcement against the people who are actually cutting the cats off regulatory requirements just displace the problem and shift the profit around.
Being a mechanic doesn't require a license although running the commercial establishment generally has some permitting requirements.