"Not a registered user" can refer to members who have cancelled their membership, or whose membership has been suspended by eBay. For example, a member whose correct contact information is no longer on file with us may have their registration suspended until the new contact information is entered. In either case, while the member appears as "not a registered user", he is not allowed to buy or sell on eBay.
Here is eBay's explanation of shill bidding...
http://pages.ebay.co.uk/help/policies/seller-s hill-bidding.html