buyer not returned an item

[itsallmine10]

I sold an item in June. The buyer said the item was faulty and  said they have returned it.

I'm now in possesion of a letter from Royal Mail proving they have not returned the item.

The buyer also had the cheek to leave a negative feedback.

[plpmr]

the neg is too old to do anything about and your response says they decided to leave the neg instead of returning it.

How can you have a letter from RM proving you did not get the item back?