Spot on, I'd forgotten that they are on the exceptions list when booking a courier. For example, from Parcel2Go it states:
guitars - maximum parcel compensation value for damages is £100.00, + up to £1,000.00 compensation for lost items. Guitars must be sent in a hard case.
So it had to be sent in a hard case to be covered, which the courier would have known from the second he picked it up, as they ask what the contents are when collecting.
Get back under your bridge troll.
Ah the good old "I'm told I'm wrong on something, must be trolling."
What you said doesn't apply at all. The seller was tasked with packaging the guitar well enough to and didn't perform.
That's all there is to it. It's got nothing to do with the costs of packing material, that should be accounted for in the price the seller asks considering they did invite buyers to arrange their own courier.
The seller is completely responsible for making sure it's protected well enough simply on the basis that the buyer can't be reasonably expected to oversee the quality of the packaging without being there in person.
So it's very simple, it's the seller's responsibility regardless of whether the buyer arranged collection or not. Had the seller arranged the collection they would have still packed it the way they did.