Do you mean how to "confirm" a Buy It Now? Most BINs these days require immediate payment (either by the seller or by eBay) and so when you click the BIN button you are taken immediately to a payment screen rather than a confirmation screen (where you would confirm that you are committing to buy the item by clicking a button saying that at bottom left).
If you mean something else we need context of what you are seeing that makes you think that "affirm" is something for making a purchase on eBay. A screenshot or copying the exact text with enough context around the phrase would be helpful for us to help you.
