I just never post outside the UK and I state this very clearly in bold in my listing. I also state that if anyone does bid from outside the Uk without checking first, their bid will be cancelled, and if (as has happened) someone bids at the last min and the winner is outside the UK, then ebay will be contacted to cancel their bid on the grounds of them being outside an area I post to.
When I first started ebaying and was happy to post abroad, it was a tiny item £2 value, to the US. It came back weeks later saying not at this address!! In the meantime the buyer had been refunded through paypal, turned out she had moved and not changed her paypal address
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There have been many occasions where people email me to ask if I will mail to wherever, I just politely email back to say 'sorry as per listing detail I only ship within the UK' most times. Sometimes if its someone in France, Germany, Scandinavia etc I check their feedback and will post but ONLY small light things so I don't get stung on postage if it doesn't turn up.
I have had at least 10 occasions where at the last minute the winning bidder bungs their bid on and then emails me to ask what the postage is to Russia or Eastern Europe. I just tell them that I am contacting ebay to have the transaction cancelled, then I offer the item to the next one on the 2nd chance.
I don't think I lose out by only selling within the UK, because although you may get less bids, you take less hits too.