I was surprised I didn't find an answer to my question while searching, as ebay items are usually a common question, but I didn't find it, so here's the question:

Are the cheap ebay catalytic converters useful, or not?
http://cgi.ebay.com/ebaymotors/ws/eB...category=33629
That one is $50 plus $18 shipping, so it's not much cheaper than one from a real supplier, but I've seen similar ones for significantly less.

I'm not in California, and we don't have smog tests, so I don't need anything to pass any stringent tests, but I want something so my conscience will be clear... and the hippies won't yell at me.
And I want it to be cheap. I'll run without a cat if the cheap ones are really bad and I can't find one in my price range.

 

That cat is junk, I had a similar and it clogged up in a month.

 

Maybe I should run without a cat for a little while until I get the tuning reliable, then think about adding one. I hear a bad tune can ruin a cat in short order. Any truth to this myth?

 

There is truth to this (running super-rich and super-retarded can destroy it) however I'd also caution that adding / removing the cat affects tune. When I first installed my MS, I built up a test pipe for the same reason as you. After I put the cat back on, I had to re-tune.

A decent cat is not much more expensive than a crappy one. The MagnaFlow 91006 is only $67 at Summit, and the 59956 spun metallic is $101.