Depends on the person who has lost it for you. A lot of things don't arrive because they fail to make the expected/desired price. If the person selling has a history of losing items, call their bluff - it has worked for me in the past. In cases where there is no proof of posting, I did it by suggesting that they look a bit harder for the coin and in return, I won't leave feedback. As long as he has proof of posting, then it absolves him of being the reason it is lost. I've had things go missing in the post from a proper auctioneer where I've been paid out, but asked if I still wanted it when it was returned to them half a year later. So technically not my property any more, but it did no harm to ask me the question. There are no rules, though I suspect that it goes along the lines of insurance. i.e. the insurance company owns any recoveries.