Silverstrad is correct.
Contact your buyer and inform them you do not ship internationally. Ask them if they would agree to a Cancellation Request. If they say they will agree, send them a Cancellation Request.
Visit the eBay Resolution Center to learn how to do so.
OR, determine the shipping costs, and send them an invoice for the very expensive overseas shipping (with appropriate delivery confirmation). Your next step would depend if they paid or not.
Above all, remember that merely stating that you ship only to the USA is insufficient to stop non-USA bidders from bidding.
• To learn how to block unwanted bidders from your auctions, see
http://pages.ebay.com/help/sell/manage_bidders_ov.html
• To learn how to block certain classes of bidders (such as bidders whose primary location is in a country to which you do not ship, or or bidders with excessive Unpaid Item strikes, or bidders who do not have a PayPal account), see
http://pages.ebay.com/help/sell/buyer-requirements.html
Note: If you block a bidder, AND select the "Don't allow blocked buyers to contact me" option, the blocked bidder will not see the "Contact Seller" link on your listings.
• To block PayPal payments from bidders who do not meet your specifications, see
https://www.paypal.com/us/cgi-bin/webscr?cmd=_profile-pref
Are those instructions understandable, or do you need a simpler explanation?
A great place for quick answers; no waiting, no voting,
"Do It Yourself": eBay Customer Support .