I did a quick check, there are currently 4100 used cats on eBay.
But it looked like almost all are unbolted, not sawed off.
So it would seem it is likely unscrupulous junk yards that area buying off the sawed off ones and either reselling or having them melted down for the metal.
If so, seems they are providing the market for the thefts...So why would it not be illegal to buy stolen CATs?
I think state laws on this are all over the place.
And of course cats are typically cut off for recycling at a car junkyard before crushing the body, because that’s the fast way to do it.
So it’s probably difficult to monitor.