My advice: Don't sell high denomination notes on eBay when those notes command little in the way of a premium over face value. It's a waste of time.
Buyers should always have the ability to return items, and that's the way it should always be. Just because eBay allows sellers to choose a "no returns accepted" policy, does not mean you get to sell anything "as is". Go ahead, keep the buyer's money and see what happens. I bet you'll have it clawed back and the buyer will get to keep your note. Then you'll be crying about more than the postage costs and listing fees.
I suggest accepting the return, providing a refund, and asking eBay to refund your seller fees. Yes, it's possible to get your seller fees back, and it's not hard.
Absolutely under NO circumstances re-negotiate the final sale price of the item. A few buyers are chronic whiners and will do anything to get a post-sale discount. That's what could be happening in your situation. The buyer paid too much for the note and is trying to weasel out. Ask him to return it, and if he doesn't, then you successfully called his bluff. If he insists on being difficult and leaves unflattering feedback, blackball him.
Also: in order to avoid future problems you are better off to state:
"The note appears to be UNC but that is my opinion only. Please view the photos and judge the condition of the note for yourself. All sales are final. Do not bid if you intend to judge the note in person and decide you don't share my opinion for the purpose of returning it."
That's great advice for anyone who absolutely wants to get as little as possible for their notes. Seriously, putting conditions in your listing like this just screams "corrupt seller" and will quickly sink a reputation. Saying that you think the note is one grade but buyers should form their own opinion tells the buyer you have no confidence in your own grading. It also sounds like a disclaimer, leaving no room for buyers to show any confidence in you. Whether you like it or not, buyers hold sellers responsible for their listings, and they will be more likely to trust someone that owns up to their responsibility.