No seller is off the hook if they packed the item poorly! The courier is there to collect, not to oversee packaging and confirm everything is secure unless hired for such a service. If I, as seller, am a silly billy and throw the bass in a $5 carry case how is the courier or the buyer responsible when the bass gets damaged on its way to the buyer? No offense but I find this simply excuses for insecure sellers to sleep better at nights. You either offer both collection and shipping or just collection.
Dan it is no demand. If not clearly stated in the for sale ad "collection only" i will kindly ask the seller if posting would be an option, again "ask" not "demand". If the seller offers shipping I will then politely ask how will he ship? In a Tesco bag, original carton, HSC? I will always pay for the shipping costs, as is my obligation as a buyer.
I am still trying to understand if there is a UK law relieving the seller from all liability when he ships an item only when the buyer "arranges own courier"?