As far as I know Royal Mail will only consider claims from the person who bought the service (the poster).
Although the Royal Mail in this neck of the woods (and Parcel Force) is very good, delivering a quick reliable service, I've found that the larger or heavier the parcel the more likely it is to bear damage. Having said that, I've never had any expected delivery fail due to damage being caused to the packaging - and I've had boxes delivered here hanging open and the contents smashed up (mostly by the non RM couriers I have to say) .
Although it's not beyond the bounds of possibility for damage such as you describe to happen, it seems fortuitous in this case that, despite the package ‘badly damaged', enough information remained to allow its return to the poster.
As this may be difficult disprove, I would take the refund but keep an eye on their future listings in case a similar item appears for sale again – for your own information more than anything else, as the vendor could argue that the item in the ‘new’ listing was another radio of the same type to the damaged one.
If something like this should happen, and it irks you, I suppose you could bring your dissatisfaction with the vendor to the attention of eBay – just in case there have been other complaints against them not evident in the feedback score.
Insofar as the subject matter of your op is concerned I don’t think there’s any point in raising a dispute with eBay as the auction was closed out with you being the successful bidder and, despite the unsuccessful conclusion of the transaction due to the failed delivery, you’ve had a quick refund of whatever money you have paid to them.