YES, if he is that interested in it, then by all means have it checked out. It maybe that 'one' coin well worth the investment.
Taking advantage of someone by selling this worthless coin for hundreds of dollars is, IMO, highly unethical. It's not something I would do. Selling this coin for anything over face value puts a nasty and long lasting stain on the entire numismatic community.
The naysayers are absolutely right. If someone wants to buy that coin from you, sell it for no more than five cents.
If you tell the prospective buyer that you believe it to be damaged, and that others with coin knowledge have said the same, and he/she still wants to offer you more than face, I see nothing wrong with that. That would be a very ethical thing to do.
I have 1972 S 1c that is still in the UC Mint cellophane. I had always thought it was the heating or sealing wheel used to seal the various cellophane pouches. I was told no added value but that never made sense to me since it had to have happened during mint ownership and packaging process. Good luck!!!
Error collectors are only interested in things that happen before or while the coin is actually being struck. Anything that happens after it comes out of the press is just damage, even if it's still in the same building.
But it is not in the coin manufacturing process, it occurs post minting. PMD is post minting damage and not considered in the process of coining, but packaging. which by the way is becoming robotic ( Mint has a video on Youtube showing this)..
There is also what i think is mint damage to the coin as well. Just to the right of the the word "cent and below the memorial". It fails me what that is and how it happens. If honestly i had never noticed it until today when i took photos for my reply...
There is a lot going on with this coin. Weird Toning, Delam, and Packaging Damage. I am thinking there might even be damage on the obverse. So i made a thread on the Errors forum for this coin and included the obverse. I cannot get a good picture of the damage while it is in the cellophane. I will not remove it either.
How many times do we need to protect fools from their own foolishness? OP did not say he priced this coin. He said a man saw it and has now made five offers each $100 higher than the previous one. OP checked here and no one could explain why someone would value his coin that highly. All that said I would have no problem accepting the next offer and using the money to buy myself a really nice coin. After the coin changes hands I would politely ask the buyer to tell me two things. 1) Why did he want it that bad? 2) If I find another one does he want it too?
Generally speaking I agree. However, I can also think of a number of instances where doing so may actually serve the greater good, or at least teach someone a personal lesson. When someone decides to sell on eBay, rent a cabinet at an antique mall, or otherwise inject themselves into the retail market, they also rightly open themselves up to the pitfalls. This is why there's nothing wrong with picking such people even when it's likely they don't know what they're doing. Perhaps it's nice not to do so, but they've no right to complain as they made a choice and must live/deal with the consequences. Similarly, if (IF) someone goes out of their way to try to buy what was not offered to them, especially by someone they assume doesn't know what they have, all responsibility rests upon their shoulders. It's the innocent folks we should watch out for. Just my most humble and respectful opinion, of course.