WARNING: Legalese ahead.
In general, the law recognizes two types of auctions: "absolute" and "with reserve". "With reserve" is a broader term than what we usually think of (i.e., an auction with a reserve amount). "With reserve" basically means that aside from the putting-up of the item there are other conditions of sale. As far as I know, the majority of states have a presumption in favour of "with reserve" auctions, meaning that the law will presume that an auction was "with reserve" unless the intent clearly shows otherwise. Section 2-328 of the Uniform Commercial Code deals specifically with auction sales and the presumption that auctions are "with reserve". Reading section 2-328, on its face it is dealing with face-to-face auctions but it's just as applicable to on-line auctions. As the Buy-It-Now feature reminds buyers each time one uses it, an eBoy auction is a "contract".
In this case, eBoy has general rules that must be complied with and individual sellers can have individual rules, such as restricting bids to the U.S. Thus, by definition, eBoy auctions are "with reserve". For this type of auction, someone can be the high bidder but the auction item can be nevertheless withdrawn if the winning bidder hasn't, didn't, can't or won't comply with the additional terms. It is up to the seller, however. If the seller agrees to waive the U.S.-only condition, the other losing bidders don't have the right to compel specific performance (i.e., let them have it).
That's one side of my brain talking (the lawyer side). The other side says that when living in Canada, there were many, many times (at least 40) when I'd see a U.S.-only item on eBoy, I'd write to the seller, I'd ask him/her/it to look at my feedback, and I'd ask him/her/it if he/she/it would sell to me even though I was in Canada. I only had one refusal. Of course, those conversations don't get seen by anyone else. I never made a U.S.-only bid without first asking the seller if it was OK. If someone does make a bid without getting that go-ahead, it's their problem. But, as I said above, it's between the seller and non-conforming bidder.