If items genuinely go missing in the post, which very very rarely they can do, then the sender can claim unto £46 back if sent normal post, this can be applied for only after 21 working days which is the PO time scale to expect an item to arrive, maximum time of course. More insurance can be bought at time of sending, i.e. Special Delivery, etc.
Just saying I sent it, end of in effect, is not acceptable, as a seller you have to take responsibility and take it up with the PO, get the money back and refund the buyer, it's all part of the process. If the seller is not prepared to do this, than that's the time to question what is going on here?
I have been here before, on both sides.