[botport]

Interesting question. Without answering the question directly of who is responsible for what I would assume you have three options.

1) No refund.
2) Partial refund.
3) Complete refund.

Not knowing more about the transaction specifics such as item value and if the buyer is threatening you with negative feedback I would advise offering a partial refund upon return of the poster. My reasoning is that although the buyer is surely disappointed you have no chance of receiving the item back in the condition that it was sold and delivered. In my opinion because of the removal of the poster from the original packaging you should not be responsible for a complete refund. No refund at all seems insensitive considering it is a reasonable expectation to receive a sealed poster in condition suitable for framing. Perhaps I am wrong with this assumption.

A possible solution would be to ask the buyer to return the poster for a partial refund with an additional refund to be sent after you relist and sell the returned item, minus your fees and costs of course. Hopefully, this type of solution would satisfy both parties.