OK, now we know what inspired the attack from them at least. What I would do is reply to the message, keeping it very professional. I would thank them for their concerns in regards to my product, but I was happy to be able to reassure them that the product was in fact genuine, and I could substantiate that documented proof.
That will hopefully shut them up, but no guarantee that they haven't already reported your item. Not much more you can do at this point, just sit and wait to see if eBay acts and removes the listing. At that point you can then state your proof to them to get that over turned.
If you feel that for any reason their own item may be fake, then you can report that for sure, although at this point I would imagine they would just retaliate and report yours, if they haven't already.
Messy, but I would suggest sending the letter and seeing if you get a response, and if so what they say in reply BD :)