The buyer is paying sales tax in his jurisdiction.
When you have more sales you will notice that your US customers are also paying tax on their purchases.
EBay assessed the tax applicable in their jurisdiction, bills and collects it through your invoice, and remits it to the correct tax office.
That's over 200 different taxation offices just in North America, because some cities and counties also have their own taxation rules.
For this service, eBay collects a fee, by including the customer's entire payment including taxes remitted, from your Managed Payments account.
