This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
Have you given them at least 3 working days to reply? If so, try using
a different e-mail service to contact them in case your messages are
being blocked by their ISP. If you still aren't able to make contact
after at least 7 calendar days, you can go to eBay's Non-Paying Bidder
Alert Form located at
http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?NPBComplaintForm . You can
file the NPB anytime between 7 and 45 days after the auction closes.
You may also wish to obtain the buyer's contact information from eBay. If
you have a current or recent transaction with someone, you can have eBay
send you their contact information comprising their name, phone number and
city/state. You will not receive their address. Keep in mind that the user
will also automatically receive your information when you make the request.
The contact request form is located at
A special note should be made regarding dutch auctions. When filing the NPB
Alert, the seller must file all NPB's for the auction at the same time. Any
NPB's not filed at this time are forfeited and the seller will not be able
to recover fees.