Sellers list their items in several ways.

Auction-style listings are common- the highest bid wins. A fixed price buy-it-now BIN listing means  the price you see is what you pay- no bidding.

Other listings are listed as auctions and have a Buy it Now price. Once someone chooses to use BIN, they've effectively bought the item and the listing ends.  Conversely, if someone places a bid, the BIN option dissapears and bidding begins. Sellers want to sell their item at the BIN price and may offer free shipping or other bonuses if you use BIN.

the take-home point:

Buy-it-Now (BIN)is an option that disappears once a bid is placed. for example: if "free shipping w/BIN" is no longer an option once a bid is placed. If you don't see the BIN option, it's not available-and whatever the BIn price was (or the bonuses offered) are moot points.

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