Please don't worry too much about that neg any more. I know it is bitterly disappointing after all this time to get one when you don't deserve it.
I am a buyer these days and I can tell you now, 100% that it would not stop me buying from you. I always check feedback and what I would take away from that comment is the buyer didn't find that particular cream effective for whatever ailed them. Perhaps they bought the wrong type of cream. That doesn't make you a bad seller. The only thing it may do is make some buyers hesitate to buy that particular cream but it depends on what is causing their skin condition.
You gave sensible feedback. Personally, on principle, I wouldn't be offering them a refund. Just wear the neg, everyone who is on ebay long enough will get some.
You could use ebay chat to ask for removal of the feedback. I am not sure if ebay intends feedback only for the buying experience these days. They have dramatically increased how much buyers can say and a lot of comments I see are half half and have comments about the product. But you can ask, it's worth a try.
But after that, try not to worry any more about it. Buyers won't.
