When you set up your Sell Your Item l, you can Block any country or even entire continents.
Like most options, this is easier on the Advanced Selling Form, which can be found through a link at the top right of the overSimplified Selling Form.
However, if an overseas customer has an address you do deliver to (eg in Canada or the USA) they can still buy.
You ship to the North American address. There ends your responsibility for delivery. Most of the Money Back Guarantee provisions are also voided when the NA delivery point (which may be Auntie Lily or a large freight forwarding business).
However there are two ways to avoid non-paying bidders.
Don't have bidding.
Most UIDs are on unpaid Auctions. So don't list in the dying auction format. Instead use Fixed Price, like 85% of transactions.
Even most Auctions have a Buy It Now option for impatient buyers.
Using Fixed Price you can opt for Immediate Payment Required, which holds the listing open and available until someone actually pays.
You can do this as each unsold Auction closes, by relisting as Fixed Price or as Sell Similar and changing the format.
These can be done en masse through your Unsold Items List.
You can't change existing Auctions to Fixed Price but you can close the Auctions without giving a reason and then relist/sell similar.
The Relist/Sell Similar form allows you to Block countries and continents.
You can also Block payments from offshore Paypal accounts through PP, but that is not very specific and would, I believe also block bidders in Germany, Australia, Brazil, etc and possibly also the USA.
