I sell to international buyers all the time and I know for a fact that it adds between 10% and 20% to the final value. I know this because I track the next highest UK bidder on all my transactions to see the effect.
I sell mostly bike items, nothing more than a couple of hundred pounds at most, and average transaction is around £50. For this kind of money, international bidders are not looking for postage fraud, i.e. when they claim not to have received the goods. If you were selling high value items, like cameras or something then yes you should be wary, but it is easily countered by simply using a fully trackable postage service which the Post Office can offer and it's relatively inexpensive. Last time I used it I sent a wheel to spain for £18
In general though fraud is an issue on eBay the majority of people buying are honest buyers; it's the basic principle that makes eBay such a success. You stand to benefit by selling to overseas buyers.
If you're still not convinced, then simply don't make the listing visible to overseas buyers. You have to allow them to see it otherwise it won't appear in any overseas listings. The only way someone outside of the UK could see the item would be if they were viewing via ebay.co.uk and even then, I don't think you can actually create an account on ebay outside of your home country.