> you cant even buy kidney friendly cat food without an expensive Rx from a vet and tons of controls

There are a few law suits over this. Essentially, the claim is that they call it "prescription," but it has no prescription medications in it and isn't approved by the FDA to treat anything. So if you have an extra bag and sell it, this is OK since it isn't actually a drug, it's allegedly a price fixing scheme between vets and pet food makers.