This is part of their blurb in the Seller centre for  Seller Protection .

We're continuing to work hard to make selling on eBay as hassle-free as possible. Some of the things we focus on include:

• Enhancing detection methods to identify and block buyers who consistently break buying policies, don't pay for items or show a pattern of leaving unwarranted negative Feedback.

So obviously it is a problem that they are working on and then in their Seller protection policy they have this link

http://pages.ebay.com.au/help/policies/seller-protection.html   to report those type of buyers to Customer

Support,(after you check the Buying practises policy to make certain they fit the criteria:

http://pages.ebay.com.au/help/policies/buying-practices.html

Were it's best to go straight to their Contact us and follow through from there,

I've made multiple reports in the last month as they seem to be popping up all over the place and it's becoming

a real problem for sellers,

So please follow through on any you come accross and "help" them get rid of dodgy buyers,