I think you're being too kind to the seller here. If he stated they were NOS and made no representations about the damage that is clearly visible, then you have every right to cancel the auction (and would be crazy not to IMO).
As others have said, the pads are worn. They have been used--well used. I've owned a pair of R10s for at least 5 years and my pads are in better shape than those. Granted, I don't use them every day or even close to every day, but I've taken them to several Head-Fi meets so I'm not the only one who has used them. agile_one had a pair of R10s that are as old as mine and even in better shape than mine, and he too, took them to many Head-Fi meets.
So lie #1 was that they were NOS. Clearly they have been used and you don't need to have close up pics of the headphone plug to confirm this.
Lie #2 relates to the "corrosion" excuse! If they were truly NOS, that means "new old stock" which then means they should be in "new" condition. In other words, they should have been safely stored in appropriate room conditions. If they were exposed to extreme heat or cold or dampness, then even if they had never been used (which clearly they have been), then the damage caused by such neglect would also disqualify them as being NOS.
In other words, you have to hold someone to a "reasonable" standard of the NOS definition and not a purely legal interpretation. Maybe he bought them for his favorite cow, Bessie, and they've been kept in a barn for 15 years. His problem, not yours.
Don't pay. Cancel the auction, send these pics to eBay (they will rule in your favor and no damage will be done to your account or reputation), and move on.
Lie #3 is the "unknown" which I'm guessing would be damage to the drivers from moisture. I just can't understand how "corrosion" could cause the flaking off of paint to a pair of headphones that have never been used. Either a) they have been used (99% chance of that looking at the well worn pads) and thus the damage was caused by banging them against something, or b) the damage that you see (from "corrosion") in an unused pair suggests that there will be further damage to the headphones that you don't see--moisture is not good for drivers.