These were printed on pulp paper, the cheapest available at the time (well, maybe newsprint was a little cheaper, but not much). There was a ton of acid residue left over in the paper, which is why these things would yellow over time. This is why when you have a particularly nice piece of art framed that you use "acid free" matting.
First Superman comic is expected to fetch millions on eBay
The copy of Action Comics #1 is said to be in the best condition of any copy currently in existence....
According to Cnet, the copy soon to be auctioned on eBay was rated by the Certified Guaranty Company, a well-known comic book quality rating company, as having a condition of 9.0 out of 10, the highest quality rating a copy of Action Comics #1 has ever received. The colors are said to be vibrant, and it even has white pages where most existing copies have long since yellowed.
This is a hoax, just like the Diaries. And for precisely the same reason. But this does highlight the first rule of eBay: "A [blank] and his [blank] are soon [blank]." There will be a short quiz at the end of the day.