I've sold the odd item that's highly sought after by the Russians and Chinese.  They often open an ID on ebay US because they know a lot of sellers won't post to their country but they will post to the US, so this is how they get around a lot of blocks.  A lot of Russians have more than one ID - a Russian one and a US one - for this purpose.

I had a Russian buy a fairly expensive item a few weeks ago.  I don't post overseas at all but they got around my block by changing their primary address to Australia before they bought, then changed it back to Russia immediately afterwards.  If they do that you'll never be able to stop them buying, but if you've got your overseas blocks in place you can cancel the sale by saying there was a problem with the buyer's address.