David
If you want to run an auction for US buyers only then you should make sure the right boxes are ticked.
This will prevent overseas bidders being able to bid.

Let me explain how it works for non-US ebay users signed in to their countries respective ebay sites.
If I log onto ebay.co.uk and search/browse, I will initially see items located within the UK. There is a button I can click to also display all items from all other sites where the seller will ship to the UK. If you dont set your auction for US only, then I will be able to see and bid on it.