Was there anything in the listing that said they would refund if they had run out of stock? The same thing happened to me, got an e-mail from PayPal stating I had been given a refund so I messaged the seller and asked why; seller said that they had run out of stock so I went back to the listing and saw a line in the listing  that said they would refund if they ran out of stock...

I hope you didn't leave the seller a Neg before asking why you were refunded..