Yes had this before. Was a skirt and they said the button was missing, it wasn't as I had checked. I refunded them and got my item back. The button had been cut off as the thread was still there and I could see it had been cut. Not everyone is honest and I guess you just have to get on with it. Sorry if that's not helpful
Usually, if you have the proof that the package was delivered, they will be on your side. Is there a weight of the package recorded on your reciept from post office? There normally is and that would definitely go in your favour.
I have a business that largely sells via Ebay. While it is possible your buyer is lying, it is also possible that the item came out of the packaging in transit, it's possible that someone in the post office stole it, or that someone else at the delivery address has taken it. The problem is that you can't prove any of it, and therefore you can't do anything about it other than refund the buyer. A very small number of my sales go missing on the post - some are clearly genuine, with others I smell a rat - but my response is the same - replace or refund. There is always a risk with selling online - hope this wasn't too big a loss.