Where an item is being sold within a Standard Auction as well as a Buy Now, if someone bids rather than offers the ‘Buy Now’ price, the Buy Now option disappears, and the item reverts to a standard auction.  You will then either have to wait for the auction to end or bid higher than subsequent bids that come in, to try to win the item.  Therefore, if you are prepared to bid as much as the Buy Now price, it is advisable to use the Buy Now options.