I have just had this issue happen to me twice in the last week, prior to that NEVER happening in 8 years. I now am starting to worry that a rival seller who bought from me, then left negative feedback when I cancelled the sale after previously blocking all their accounts (I noticed the contact details were the same number) is getting friends or family to buy. 

I am wondering what angle they could be trying here if so? So far, they have not opened a claim, one of the parcels only arrived back this morning after 4 weeks waiting to be collected. 

I am wondering if anyone has any advice on how to proceed both with the dealing with the returned to sender issue, and also what angle any rival seller could be trying to take here if that is what is happening?