I have received a package from TomR92 recently and can vouch that it was well packaged and he had made a good attempt to disguise the contents.
I have also worked for an agency which supplies temporary delivery staff to royal mail, although the security checks are stringent enough but cant account for someone who happened to see an opportunity and took it, especially nowadays where we know people are struggling to make ends meet. 2 years ago I had post go missing at Xmas and my details were used to set up false bank accounts and take out loans in my name. Generally my postie is good but in times of 'Xmas Peak' they have to rely on temporary staff to try and meet demand.
Just one of those things unfortunately. I hope both buyer and seller get this resolved and are compensated adequately.