Buyers can't get negative feedback, ever, since about 2006. So 100% positive buyer feedback is a useless metric. You can, however, report to eBay that you think a particular buyer is acting in bad faith. Then they'll claim to keep an eye out for that person. Presumably, if the newly created accounts are related to an older account / credit card / IP address, and a pattern of abuse emerges, they'll theoretically possibly once-in-a-while do something about the guy.
At least, that's the theory. In reality, it usually means that if your roommate ever starts selling on eBay, you'll probably be suspended for suspected shill bidding.