A bit of a longshot, but... Is it possible that the buyer has already paid? If you log directly into your PayPal account, does it show a payment that's not (yet) propagated a notification to eBay?
(On a side note, you have to love those mangled English-as-a-foreign-language error messages seen here. That one in particular has been around for years. It gives a great insight into the utter lack of testing, peer review or Quality Control that's actually completed on eBay website software before the code is pushed into Production use. 
)
