to sell the cat for precious metals is really no work at all.
Step one: remove cat from Car. (this could be part of another process - say a high flow cat install or other exhuast work)
Step two: drive to a metal recycler and ask them what they will give you for it.
Step three: if they offer enough cash take the money.
Step four: drive away.
It is true the prices change day to day, when I took mine off they were going about $25, 9 months later I wanted it out of my garage and took $50, some guys here have been paid over $100. There was a time that thiefs would take a cordless sawsall and steal them from parked cars, the older GM Trucks as I remember had more metal then most are were the main target.