I am a seller who ships to the United States only. My last sale was to a buyer with a Washington state address (the registered shipping address provided to me by eBay). Yet eBay charged me an International Fee "because the buyer's registered address is in Canada." What goes on here?

Because the buyer is registered in Canada and probably used a Canadian based credit or debit card you were charged for an international sale regardless of where the item is being shipped to.

eBay charged me an "International Fee" for a United States transaction. | Payments

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eBay charged me an "International Fee" for a United States transaction. | Payments | eBay Community