During the listing process you get to the postage section. In the International section you have three options to select.

1) No international postage

2) calculate postage

3) flat rate to all customers

Select your postage cost method. Usually calculted for parcels or flat rate for large letters. ( you can also buy international satchels for some destination postal zones )

Once you have selected this a new box appears titled " Posts too ". Choose custom location.

You then have a series of destinations appear with check boxes. Simply check the countries you wish to post to and job done. I think if you want to get even more complicated there is an option to mail to everywhere except specific locations chosen.