Dont be afraid to buy from international sellers! I personally have bought and sold from and to sellers and buyers world wide, from America, Japan, Italy, and Australia.
If a seller has listed that they are willing to sell world wide but have not stated a postage cost, drop them an e-mail and ask what they estimate the postage will be, one seller re named it for me as a guestamate! Some sellers will check using a web page for postage costs, go with your gut feeling, if it does seam totally over the top then it usualy will be.
Shop around ask a few sellers with the same item and judge what the postage really is, remember that there is different ways in which to ship internationally, boat, air, land, which all take time to arrive some slower but often cheaper than others.
Personally i wouldnt buy anything too expensive or breakable from an international seller, for obvious reasons, its too much hassle to send back, and you end up out of pocket.
Watch there feedback!
Sadly some buyers buy there feedback, in order to fool you into thinking they are fab sellers, when really they are just out for a quick buck. A good way to spot this is to scan there feedback, have a look at what they have sold, and who has given them feedback, is there feedback all the same or from the same person over and over again?
Dont get too paranoid, lots of buyers tend to buy from the same sellers over and over again if they are happy with there service and products, usually if it looks fishy it usually is.
For selling to other countries, visit the post office web site or which ever postal service you intend to use, and get a few quotes at different weights, times, services, so you have a rough idea of what the shipping will cost.
Also a good idea is to add to your listing some services for the buyer to choose from, and state that it should take 7-14 days to arrive (maybe longer to cover yourself) depending on postal service, its no good saying it will be there in a week when you have sent it by boat and not air mail/air sure.
And remember your promise, if you say you'll post it next day, then do so, if you cant make it out till 2 days later tell your buyer, they'd prefer the truth, and it will get you better feedback ratings for your honesty.
Also be prepared for a language barrier, not everyone speaks, or indeed writes English.
Try to stick to using Pay Pal, its the safest way, both for buying and selling.
Hope this helps a little, remember shop around for the best bargains, use your watched items section, dont get ripped off , and enjoy e-bay!