Don't just post it return to sender, open an ebay dispute then the seller should have to pay for the return post. Any decent seller for an item that damaged would refund you in full and not ask for it back, they won't be able to sell it again as being in good condition so why they want it back is beyond me un;ess of course they sold it at way above cost price.

They were also very stupid sending it in an unregistered envelope so would have no tracking and no seller protection, Many people would, if it looks as bad as it sounds would just go for an item not received dispute.