You raise a good point, and honest manufacturers could (and may already) do this.

Congress has not enacted legislation because no one is pushing them to. Similar legislation for coinage was urged by the U.S. Mint. Why didn't they include casino tokens? Because the U.S. Mint doesn't make tokens so the mint did not push for it.

Why won't eBay do it? They will, but once again the issue is how they can spot fakes without opening themselves up to unfair competitive pressures. In other words, how can they be sure that the identification of a fake is reliable? Do you accept any person (buyer, seller or bystander) sending you an e-mail saying "it's fake" and then you pull the listing?

Other than an actual copyright holder's complaint, I am aware of only one area where eBay has imposed requirements on authenticity. Again, that area is coins: only "approved" slab companies are approved for listing, at the behest of the coin industry as a whole (with the legitimate certification/slabbing companies no doubt funding the movement).

If I see something I think or pretend to know is fake, why should eBay believe me? If my competitor thinks I have something fake for sale, why should I let eBay pull my listing?