'Buy Now' purchases an item immediately at the lowest List, and cannot be changed to a Bid after the fact. There are a few indicators to make you aware that you're about to purchase an item as opposed to placing a Bid for it:
The text "You are buying from the lowest list" appears below the price quoted for the item
The button in the bottom right-hand corner of the screen will say "Checkout" as opposed to "Review Bid"
The Item Price box has been filled in automatically, as opposed to the blank text box
Once you've clicked "Place Order", the sale goes through and cannot be changed to a Bid.