Scrap dealers buying used converters are supposed to get a copy of the seller's ID, a photo of the seller, the license plate number of the seller's car, and video of the transaction itself. Payment by check must be mailed, and only after a mandatory three-day waiting period.
However, plenty of out-of-state scrapyards will happily buy a catalytic converter from you through the mail with just a photocopy of your (or, really, anyone's!) driver's license.
Catalytic converters work their magic by passing emissions through a honeycomb impregnated with a few grams of platinum, palladium and rhodium, in varying proportions.
You're familiar with platinum, so you won't be surprised to learn that it's currently $966 an ounce. However, you might not have known that an ounce of palladium is $2,246—and you probably won't even believe that an ounce of rhodium goes for $13,700.