There's a bit of a problem shipping to the US right now. There's a tariff to be paid when shipping the item, and eBay's systems aren't geared up to do this yet in all categories so their Global Shipping Programme has been suspended for the US.
Your seller could set up a higher Buy It Now price for you to incorporate the extra fee (10% if UK, 15% if Europe plus an admin fee of between 50p - £1.50) and set up their own shipping method.