If it is still pending, contact the seller. They may be new, and not realise they have to accept the payment, or they have the wrong email on their listing.
Advise them to check that it matches 100% with the one they have in Paypal. Ask them to click on the item, then on Sell Similar, scroll down the lsiting to the email address there. Even a comma there instead of a dot will stop payment going through.
