mhm, I must say I don't think that a slanging match (name-calling etc) is necessary in feedback at all: positive, neutral or negative all needs to be factual. Having said that if some-one is in a bad mood, it might occur!.where they actually called the seller rubbish.
If they leave bad feedback for you, I would simply respond to it factually: how you tried to resolve, etc. If you suspect there is something funny going on, after the tracked re-send etc, let support know.
It would weed out any fraudulent "not received" claims... and show up at a glance if something genuinely has gone walkies. I do this with most things, anything costing over £1 (item cost + P&P inclusive) now.why do you not do this with the first item ?....it would save you this kind of problem , or it should
I still like to think most people are honest, but with times getting harder, well...
Really, not a lot can be done until such time as you hear from the buyer again. I would make sure that you send them the tracking details for the replacement.
Also, just a thought, even if not getting a tracking number for things you send, always get a certificate of posting, because Royal Mail will not do anything about a postal loss claim if you do not have the minimum of a certificate of posting.