Buy It Now means buy it NOW, not wait and see how the bidding goes then decide. As such the Buy It Now option ends upon some point in the bidding being reached (if it didn't it would act as a cap on the auction price, which it was never intended to be).
In the vast majority of circumstances the BIN option ends when the first bid is placed, with the following 2 exceptions:
1. If there is a Reserve Price on the auction, the BIN will remain up if the Reserve is not yet met.
2. In 4 "Sticky BIN" categories the BIN will remain up if the "current bid" is less than 50% of the BIN price. This is a relic of an experiment eBay did to determine whether to keep the BIN up longer than the first bid several years ago. They tried it in different categories where they tried different percentages of the BIN for the kill point. When it was in the 4 categories at 50% they decided not to implement it further and were going to remove it from 3 of the 4 categories but ran into technical issues and so just left it on all 4.
