Ebay has changed its feedback procedure (for the better) in the last several years. A neutral rating will neither help you or hurt you.
The would-be buyer probably did the best he thought he could; he was justifiably disappointed that what he bid on was not what he thought it would be.
You also did absolutely the right thing by refunding his money right away. He may not have mentioned your speed in refunding, but if you had been slow about it, you may well have received a negative rating instead! Think about the same thing happening in a physical store; if "I'm returning this product and I want my money back" does not receive an immediate response, you don't want to shop there again, right?
I, as an eBay buyer, might look through your feedback and see a neutral rating, but I wouldn't worry about it as long as I didn't see any negatives. I wouldn't wonder about your products or your integrity as a seller.
Don't stew about it. You inadvertently made a mistake in listing, and you know you'll never make that one again! :^)