I personally don't know how much a seller can protect him/herself when dealing with overseas bidders and paypal. If nobody here has a good enough answer for you, you might want to try asking your question in the Answer Center on eBay or asking customer service on paypal. Paypal usually gets back to you right away and most of the people who respond on the Answer Center usually know what they're talking about. You might even be able to find your answer there without even posting.
And here's a link to block any bidder: http://pages.ebay.com/help/sell/manage_bidders_ov.html