Possibly they may have their email filters set to exclude your ISP without knowing they did so. Try going to Yahoo Mail or any of the other free mail host and setting up another e-mail account. Also remember that they may be trying to e-mail you and their mail to you is getting bounced. Keep trying and if you can t get them by e-mail get their contact information from Contact Information and call them. If all else fails post a neutral to their feedback account saying that you are trying to get in touch with them and make sure you use the auction number.
People do have emergencies that come up so be patient.
Don t post a negative feedback until all other avenues are closed and give it several weeks before you do. If you have been defrauded on an auction you may go to Safe Harbor Insurance Guidelines and see if you qualify for reimbursement by LLoyds of London. If the amount of money is small the odds are that you aren t going to get it back. If the amount is large you'll also need to see your local State Attorney for information on how to proceed. If someone request your contact information ebay will automatically give it to them.