Happens all the time.
However, you are not SOL.
Just go to the credit card you really paid with (Paypal is just the mediator) and call their 1-800 customer service number. Ask about a chargeback.
The rep will need some information about the transaction to process your refund. Dates, amounts, transaction numbers.
But even credit cards have deadlines so act today.
Bonus, your smartypants seller won't know what hit him.