If you have found out off a buyer that an item you posted to them hasn't arrived or it has arrived broken you can claim the money from royal mail. You can download claims forms (P58) from this link
www. royalmail . com /portal/rm/jump3?catId=500185&mediaId=400144
(Please note ebay wouldn't let me add the link so I have broken it up. Just get rid of any spacing between to go to the site.)
It is simple enough to fill in and doesn't ask for loads of paperwork and takes anywhere between 2-4 weeks to process. Royal mail will try to find the item for you. If it isn't findable they will refund your money. Please be aware that if you are a buyer and you are trying to take advantage of this and claim your money back even if you have received the item royal mail will know you have it. EVEN IF IT IS POSTED 1ST/2ND CLASS. You can then face a court order.
Sellers usually have to claim for the package but either party can do it. Just make sure you still have the proof of postage receipt the post office will have given you and proof of how much the item inside costs.
Royal Mail also refund money for any items broken in the post excluding ceramics unless they are sent special delivery.