Require the scrap dealer to demand from the scrap seller that he document every piece of scrap that he is selling to the dealer?
Lets see, my truck has 300 lbs. of rusted out steel farm equipment, 125 lbs. of old aluminum siding, another 125 lbs. of old iron railings, and 15 lbs. of copper tubing...........
So how much documentation would you require for that MrShoop?
It is just preposterous to say that there is no legal, or moral obligation for a buyer of anything to know that what he obtaining is legal. Let me give an extreme to try and clarify:
A sweaty disheveled man runs into a pawn shop. He pulls a purse from under his shirt that has had its straps slashed. He rummages through and pulls out an iPhone with a pink case, holds it up and asks, "how much will you give me for the phone?" I know you wouldn't say that it is ok legally or morally to buy this phone.
You can look at this three ways - a buyer either has zero responsibility, some responsibility, or an absolute responsibility to make sure he is buying legally. You seem to be arguing for zero responsibility, I'm saying you have some responsibility. However, when you are in a high risk business, then your responsibility goes up. Sadly, scrap metal (especially in an urban area) is more and more a theft driven business.