Don't think you'll find anything on car-part.com. Pretty sure there are federal laws that prohibit the sale/installation of used catalytic converters (typical EPA nonsense). So your best bet is gonna be finding a private party getting rid of a set.
I am well aware of all the EPA rules/laws regarding the resale of catalytic converters as I previously owned a large salvage yard and sold it in 2015. I also still have my own personal EPA id number assigned to me.
By EPA mandate, when a cat comes off a car it cannot go onto another car until the cat has been "re-certified" to prove that it is still able to "clean" the throughput air to the specs that it was initially certified at. I have no idea who you would go to to get a catalytic converter 're-certified". When I buy cats from people they don't go back onto cars. I do all the upfront legwork and sell them directly to a company that cuts open the "can" with very specialized equipment and harvests the precious metals from them (including the dust!). I eliminate all the middlemen.
When you walk into a salvage yard to sell a cat you are getting the lowest price possible because the typical yard owner/buyer doesn't know the value of a particular cat because there are about a hundred different styles. In order to protect himself, he will offer you a very low price because he feels this protects him from making a mistake. The other huge consideration is that the value of the cats can change on a daily basis due to market swings of the internal precious metals: Platinum and Palladium. They are commodities, just like gold, silver, corn and oil, that are traded world wide and can fluctuate quite a bit. I have seen the value of one style of cat I deal in ( 6th Gen. ZL1 Camaro Primary cats) drop as much as $100 each in a week. Occasionally I need to provide a cat to this company for them to analyze individually because it is so new that it is the first one they have ever seen.
The bottom line is that there are many, many variables when you want to sell the cats from your car, but selling them on Ebay or bringing them to your local junkyard may not be the best way. Thanks, Kevin C.
It appears from your additional information, that your intention for the desired cat was business, not getting a particular car back on the road like most folks who stop by here. Had I known that, I would not have wasted my time, or yours.