Dispute resolution for Paypal is not on the side of the buyer, unlike with a credit card. So you can't just complain and force the seller to prove he's innocent.
You need to first contact the seller and ask for the refund. If he will not give it to you, then you could start a dispute resolution with PayPal but they warn that the process may take 30 days. Also, unless he's required by law to refund (due to defective product, etc) then there's nothing PayPal can do if he refuses to give you a refund.
By the way, if you deposited money via check or credit card into your PayPal account to pay for it and you then cancel the check or reverse the credit card transaction, PayPal will simply subtract it from your PayPal account balance, because technically you paid it into your Paypal account.