Pop down to your local PO, they will give you a Leaflet with all charges, rates of compensation etc.
Case in mind.
I had a rear sprocket carrier & a sprocket up on ebay last year, a guy one it for around £50 & sent me a cheque, once the cheque had cleared & parceled up the goods & sent them via the PO, for some reason the goods never got to the buyer. After a week or so the buyer contacted me saying the goods had not arrived, I replied with the sending dates plus copies of the proof of posting, this went on for a few weeks. I then filled in the lost in post form, gave all the details, copies of emails, proof of posting, cost of goods etc. The end result was compensation of £30, I sent this to the buyer. The buyer was happy enough to leave positive feedback.
I guess the moral is to make sure the seller covers the cost ofextra insurence in the cost of the postage.
Chris.