I hate to say it but the seller may have to refund you per eBay policy - and rightly so to a degree.
I think it would be wrong to expect the seller to be out the sheets and the money too on someone else's error.
They don't have big money or buying power behind them like a big name like Nordstroms. And out of that money your seller paid for sheets,PayPal, & selling fees to boot.
If you dispute it with PayPal the first thing they will do is look at the delivery confirmation that shows it was delivered. Why should an innocent party, you or the seller, take a hit? Why not put the blame where the blame belongs??
Is there a possibility the package was left with or delivered wrongly to a neighbor or maybe tucked away in a obscure spot? Did you by chance confront your 'new girl' carrier about this?
Really stinks to go through all this frustration for packages but I think we've all been there more than once.
Then going to the post office for some answers and having them treat you like the bad guy adds insult to injury
Hope it all works out with minimal aggravation and hassle.
I would suggest posting your situation on eBays Seller Central forum and seeing what they have to say about it.
I have read that on occasion Paypal will foot the bill (hopefully not your seller) when buyer or seller is not at fault.GL