In the shipping tools select worldwide under domestic check flat rate shipping, under international check will not ship international. In the domestic shipping services select what type of shipping you will be using and put 0 in the cost box.
Then in your descripition state if outside the U.S email for shipping costs.
Your listing will then show free shipping in the U.S and that you ship worldwide and the descripition will let buyers outside the U.S know to email you for shipping rate.
That is how i do it.
Here is a link to my ebay store if you want to see how it looks.
Hope that helps.