Why is someone allowed to leave negative feedback after they have lost a case against you?

[raycarp.0646]

I had a situation where a buyer claimed they had not received an item and raised a case against me, claiming that they had received a damaged envelope. I send them video proof of the item being posted and they admitted that I was not at fault and lost their case ( the way the coin was enclosed in cardboard in the envelope meant it was impossible for accidental loss ).  They would not provide evidence for me to claim from Royal Mail and refused to claim themselves. 

They've been allowed to leave negative feedback for me but I can't now leave negative feedback for them to warn other sellers about the theft that took place.

[arkwebus]

You were lucky!  The buyer opened a non-receipt case and - as you could show delivery - lost the case. 

There is absolutely no requirement for the buyer to have sent you any photos.  He should have  opened a "not as described case"  [ordered a coin in an envelope and got an envelope!]. 

Subject to time limits he still can [but I doubt that he will know that]

You then send him a label to return the envelope and on return you refund him.  The process is automatic.  No one says it was your fault or that you did not send it simply that the buyer is entitled to a refund.

This is what Royal Mail says about sending coins:

If you’re sending valuables, money or jewellery in the post, you should use Royal Mail Special Delivery Guaranteed®. This lets you to claim compensation if the post is lost or damaged. Valuables sent using any other service aren't eligible for compensation.

@raycarp.0646