Shipping to USA was suspended a few weeks ago, discussion thread about it below:
https://community.ebay.co.uk/t5/Seller-Central/Ebay-suspend-all-UK-Global-Shipping-Program-purchases...
The latest that eBay have said on the matter is:
Select GSP listings have begun to appear for US buyers, allowing some UK sellers to start seeing sales to the US. We will continue to work on ensuring that GSP service coverage is maximised, as customs requirements evolve. Until then if you are not yet seeing GSP coverage for your item and wish to list items for US buyers you can specify a shipping service to the US.