No, costs extra from the UK to list in the USA now as it is considered a seperate listing.
But for my penny worth, I have only ever listed on the UK site, and buyers from all over the world have had no problems seeing, bidding and paying for my items.
Afterall they can look at the UK site directly and bid.
I believe the categories differ between countries but then again a buyer who trusts that a seller has listed in the correct category and uses category specific searches will miss out on a bargain.
Many sellers particularly of collectibles and simply old stuff they come across may not even know what they have. Some just make a mistake and list in the wrong category.
Which site you list to mainly depends on your items and the target customers for them e.g. celebration date gifts which can be very country specific.