From my lawyerly perspective: the firewall tag is not illegal to sell on its own. But if the bozo you sell it to rivets it onto his 6 cyl Camaro and then proclaims it a Z-28 complete with his own handstamped DZ engine block and sells it to someone, you are guaranteed to be dragged into the subsequent lawsuit when the new owner figures out that his car is bogus. You may end up having the claim against you tossed out as a semi-innocent seller of the tag but not before you have spent about $20,000 in legal fees for an attorney who speciallizes in Federal law.
