Having had *far* too much experience of parcels going missing or getting damaged over the last few years, I know that either the sender
or the recipient can make a claim when things go awry - but whoever does it also needs the original proof of posting to enclose with the claim form, so most logically it's the seller that instigates the claim.
If the seller is playing fair, neither of you will have to cough up the cost of sending the parcel back to sender - Royl Mail will provide an address and code to stick on the front of the parcel for their inspection, and this can be sent by either party.
Edit: Ah, Martyn beat me to it.
