You don't find out what the issue was. None of your business basically. It could be as little as having an expired credit card attached to the account, or it could be selling stolen goods. Irrelevant.
Open an Item Not Received Dispute
http://resolutioncenter.ebay.ca/
Tell the RC clerk (it's all done online) that the seller is Not A Registered User (NARU) and this will expedite your refund.