I saw an auction end recently for $100 when I would have paid $250 for the card if the seller shipped internationally.
What many sellers fail to understand is that, if they shipped internationally, they would open up their auctions to a global market and thus get a much higher final sale prices.
Sure, they might get scammed once every thousand auctions, but i) the higher sale fees should more than compensate for that, and ii) there are scammers in every country, including the USA.
Also, most international buyers are willing to pay for insured/tracked shipping if its a high end card, which means there's no risk for the seller anyway.
Collecting high-end cards of NBA legends: http://will23c1982.smugmug.com/Sports/Collection