There are a couple of valid reasons for restricting one's auction to bidders residing in one's own country. It's not always xenophobia (you're looking for education, right?); it can be just a matter of practicality. However, I entirely agree that a seller should be polite in saying "No international bidders, please" or words to that effect. After all, it's bad business to risk offending potential future bidders who happen to reside in other countries.