Thank you all for your replies. Very much appreciated. I had a nice cup of tea and have calmed down.

I've refrained from replying to the feedback until I'm calmer - but when I do it'll be something factual like "item sent [date], obviously lost in post, buyer fully refunded".

I agree - defamatory comments in feedback are against eBay policy. If she'd just said "I never received my item and I'm not happy" - okay, that's fair and I wouldn't be as upset. But to accuse me of ripping her off - which literally means to scam or thieve - is untrue.

However, I expect eBay to argue that the buyer felt that way because they paid and didn't receive their item, but worth a shot.

Checked her feedback left for others: mostly positives, with a few negatives thrown in, but not enough that eBay would consider it a pattern.

Kicking myself because I've had trouble with this postcode before and I usually send EVERYTHING to that postcode via trackable methods, but because it was cheap and a PO Box at the post office, I did not. Ugh.