Used secondary cats worth anything?
 

Pulled my secondary cats to install test pipes.
Should I just hold on to them or is there a demand for these?

Hold on to them as you may need them one day for smog testing

They do have a value, even as scrap. They have platinum in them that can be recovered

I have heard this before. Any idea roughly how much the platinum is worth? And would pay me a scrapyard?

Check with a muffler shop...they usually have someone that comes around and collects the scrap exhaust stuff.

Open them up, take out the core, maybe you can get some $$$ for it at a recycling/reclamation center, weld it back together for stealthy but heavy'ish (unsure how much weight the cat block is) cat delete mid pipes... With built in heat shielding for your inner CV Boots :)

Do a web search for recycling cats

$25-50 depending on what scrapyard you take them to. Helps to cut off the tubes too.