Not sure where I read it/heard it, but I think it's unlawful to sell a secondhand catalytic converter. AN easy test would be to call a salvage yard and ask the question; they'd know.
Compared to the cost of OEM, I'm certain you'd be bucks ahead by buying aftermarket high-flow catalytic converters, and having a local muffler/tuner shop to make up a new mid-pipe section.
1957 Austin-Healey 100/6 Wine Red
1961 McCulloch R1 go-kart Screaming Yellow
1995///M3 Coupe Dakar Yellow Eurosport Twinscrew S/C
1999///M Rdstr Cosmos Black Eurosport Twinscrew S/C
1999///M Coupe Estoril Blue Eurosport Twinscrew S/C
2001///M Rdstr Steel Gray
2011 X5 35i Sport Deep Sea Blue/Cinnamon