This is also a dangerous time of year for me too, I believe its because people are dealing with their Xmas bills aftermath or maybe there's not enough sun!
I've also collected a couple recently, from the same fellow who I haven't been able to stimulate any conversation, resolution with (I've not got a "full set" from him, several positives, a neutral and a negative).
I have a standard note I wrote that I send folks who neutral or neg me that offers the same resolution options I'd have offered if they'd have talked to me first and asks them to reconsider their feedback after we resolve it. I "recover" (as in they revise) about half the time which is worth the effort. About two days before this "full set" fellow I had a neutral from a different fellow that I was able to recover.
Of more concern to me, I am selling a lot more to overseas folks who clearly do not speak english and this is increasing my risk as I do need folks to read my listings especially for some of the packaging styles I use (I suspect this is the problem with the "full set guy"). I will have to keep an eye on things and perhaps adjust my selling style again....
The good news is that with luck people will be generally happier in another 4 weeks or so!!!
Incase anyone has any suggestions, here is my standard style message I send.....
Hi! I see from the (neutral/negative) feedback you left me that you were not happy with the (whatever they bought) stamps. I am sorry to hear that they were disappointing. You do have options to resolve this, if you return them intact you can receive a full refund, or a partial refund is another option as well (depending on what they got they may have additional options here too). If you would prefer to go the partial refund route, I would look for your help to identify the appropriate refund amount given the issues you've identified. It is unfortunate that the first news I have of any problems is a (neutral/negative) rating, which is damaging to my reputation, before I've had a chance to know about it or to try to work with you to resolve it! I hope that once this is resolved you will consider revising your rating. Let me know how you would like to approach resolving this and have a great day!