Don’t. It’s against T&C. Just let the auction run and ignore the emails. IMO. And i do sell on ebay occasionally.
There is a way in Ebay to list a Buy It Now price on an item that is also under auction, and if someone clicks that button to pay the Buy It Now price it garners the item at that time. I have no idea if Ebay will allow an auction to be revised like that, or ended early and re-launched with a Buy It Now price.