Bump for you.
Just wondering, why do you want to risk buying a used cat that could cause you to fail smog?
Just go to any muffler shop and buy an aftermarket one from them for under $75 (Magnaflow or something) and they work just fine.
It's illegal for shops to sell used Cat in Cali, so even if you can find one from someone, very few shops will actually put it on for you.