I have sold a few CPUs and bits on that famous auction site and they were either shipped in those hard plastic cases in another box,the original packaging in another box or in a small anti-static bag between two pieces of foam in a reinforced box with extra padding. The main thing is for the CPU not to be free to move about since lots of couriers are very time limited and will not be treating the packages that well. It only takes one of the little surface mounted bits on the CPU to be damaged for it to potentially not work,plus a hard jolt could actually damage the edge of the PCB(probably more cosmetic though).
Plus,remember a fair number of AMD CPUs have pins on the CPU,so that means you need to even more careful since they are **** easy to damage if the CPU is not immobilised properly. The AMD Ryzen chips are not LGA,so I assume the OcUK pre-overclocked bundles for AMD CPUs,will be somewhat better packaged.