I used to work at an exhaust shop. We sold catalytic converters to a scrap yard (we also had a separate dumpster for scrap metal that they picked up weekly as well). We needed to show receipts for purchased cats that matched what we scrapped, so if we scrapped 50 we needed to show we had sold at least 50. Although the main reason for that was environmental, it was to prevent shops from removing converters and straight piping the vehicle. However, it also did help to prevent theft, although I don't know if the yard checked thoroughly for non-commercial customers.