When listing an item, you can optionally specify a BuyNow! price, which will be publicly displayed in the item listing. If you specify a BuyNow! price, any bidder can use the BuyNow! option to purchase the item immediately at that price, ending the auction.
If you also set a Reserve Price on an auction, and the bidding meets or exceeds the Reserve Price, the BuyNow! option is disabled at that time. If you do not set a Reserve Price, the BuyNow! option will only be disabled when the current bid meets or exceeds the BuyNow! price.
Note: The BuyNow! price does not affect the minimum amount at which an item can be won in an auction. To guarantee receiving a certain amount for an item, you must specify a Starting Bid or Reserve Price.
For additional information, see:
Reserve Prices and Starting Bids
Fixed Price versus Auction Listings