As far as I know, neg buyer 1 can't find out who the other buyers are to contact them, so the 2nd neg opinion should be independent?
I am no expert in the stuff you are selling, but do you know what might be making these folks think that they are fakes? Neg buyer 1 had made mention that the packaging was different? It doesn't matter if their opinion is right or not, it matters that they have somehow come up with it and if more buyers have the same opinion it is going to further damage to your ability to sell these. To my mind this is your biggest problem at this time.
If I was faced with this situation and I wanted to continue selling this stuff, I'd add extra information to the communication to the buyer at the time of the sale and again along with the item itself when it is shipped (like a piece of paper attached to the plastic bag or whatever that it is contained in) that helps them understand that it is legitimate - again I am not sure how you would do that for this material. I would also probably put something in that reminds them of the full refund if not satisfied policy you have.
As a note, your feedback response to neg buyer #1 was AWESOME! If I was looking to buy an item from you and read that negative and your response, I would know that you try to resolve problem situations, which is what I as a customer want. I always check negs/neutral responses from sellers before I buy.
