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Based on my understanding of the Global Shipping Program, there is nothing special you need to do to ensure that returns are not offered to international buyers through the Global Shipping Program.
As described in the topic called “How are returns handled for items that shipped through the Global Shipping Program?” in the “Invoicing and returns” section of the eBay Help page located at http://pages.ebay.com/sellerin.../faq.html#invoicing:Returns from international buyers can't be handled through the Global Shipping Program at this time. Therefore, the returns policy that you specify in your listing applies to your buyer's purchase of the item. You may specify an international returns policy in the "Additional returns policy details" text box. For complete details, please see the Global Shipping Program terms and conditions.
Therefore, in order to offer returns to your domestic (but not international) buyers, you should just need to choose “Returns Accepted” from within the Return Policy section and clarify that you are unable to accept international returns in the “Additional return policy details” field below.
If you are seeing another way to accomplish this on the Sell your Item form within your eBay account, please contact our Customer Support team directly from under the “Help” tab within your account and we will be happy to discuss this further with you. I reviewed my own eBay account and was unable to see anything like this.