I think some people assume it will be more complicated.
Some may post here that they don't post outside the UK anymore as they have had problems and it has put them off. I personally only have problems with Buckinghamshire - and I can't block a county! ;)
Some also get a shock when they see how much something will cost to post, but basically, if the buyer is prepared to pay that postage cost then there is no problem. Stating the postage cost to overseas on the listings makes it all easier (less questions etc) but many prefer to omit this information and wait for enquries - i personally think that this can put a buyer off.
It's a shame more don't sell worldwide, especially clothes, as there is a big market for clothes in some countries. I only sell books, but I do sell a lot to the US and Australia.
I'm in Australia build and sell musical instruments. I only sell and ship (by Express Post International - a tracked service) to Australia, the UK, the US, Canada, Japan and New Zealand. I found that non-Brit Europeans and Scandinavians expect Mercedes quality at Kia prices. They know Paypal and eBay will always side with the buyer so if a customer wants to make a return I'll get a Paypal chargeback and have to pay return post, so they're effectively betting I'll just take the loss and move on! One Dutch guy complained about a lap steel guitar I'd sold to him, said it was unplayable rubbish and did a Paypal chargeback. A month later there he was on a YouTube clip playing his "new" lap steel guitar. I sent the details to Paypal and they wouldn't budge. Word spreads amongst powersellers, so he's now on a lot of banned bidders lists, but I was still out of pocket just over AUD$400.00 After the 4th such episode I decided to limit who I'd accept bids from.
As LilQueenie said - postage options offer limited seller protection.
I sold a mobile phone to someone in Malta, sent it by International Signed For as I thought that would protect me (item sold for more than £100.)
Buyer promptly claimed non receipt and claimed payment back through paypal. I had no luck claiming for the 'lost' item as Royal Mail then said ISF only tracks the item until it leaves the UK and could not be traced all the way to Malta. Grr. Should have read the small print. I no longer sell to overseas buyers.
Royal Mail Airsure tracks the entire delivery to final destination with signature, its £11.50 to EU as 'small packets' or £17.50 to USA, Asia etc. Its the only postal service I offer to overseas bidders for my protection.
Oh and theres a useful table on the RM website of which services are tracked only to the point they leave the UK or tracked to their final destination. Can get quite expensive but it offers sellers some protection.