Like you I would only sell to domestic customers. I learned very quickly that was a mistake. As of now my international sales are greater than my domestic ones. I would be out thousands of dollars if I never sold international.
When you include international shipping in your listing, be sure to add in a default handling fee. I like to use $3.99. The buyer does not see this amount, it is added to the shipping cost before the buyer sees the total S&H amount. Meaning, if the shipping is $10.00 the buyer would see $13.99 as the shipping charge. When creating your listing, have the length, width, and height of the package, as well as the total weight. The total weight includes the box, product, packing materials...everything. Select Worldwide in the international shipping section, and enter all of this information there. If the weight is over 4 lbs. or if the 3 sides total more than 84" you cannot use USPS first class international, you will have to select USPS priority mail international as the shipping method (parcel post is very slow). PMI gives you much better tracking information. So if tracking is important to you, then definitely use PMI.
Once you've setup the international shipping, anyone outside of your domestic area will automatically see what it will cost them for shipping (including your default handling charge). When they make the purchase, the shipping will automatically be added to the sales total. Be sure to use the eBay shipping label service, it's less money than going into the post office and the tracking number is automatically sent to the buyer. Having a digital scale is very important if you want proper postage on your packages.. I picked up a real nice one for about $20 and it has proven to be invaluable.