Yep, it's a very good idea it actually came up on one of the 'major' gen forums also recently as it was a noticeable issue with the sales sections. Not sure how that went of even if it is easy to implement but it allows people the freedom to look if they want to but it gives more clarity to how open a seller is to posting overseas.
It could take the form of 'Conus only' or Conus preferred' or 'Worldwide Ship (see details)', the same could be applied to EU type sales as some wont ship outside EU.
At least it would offer some further clarity and if the seller wants to add further details/exceptions then they can be added in the listing.
Good suggestion qed!