OK, here's how it works:
A seller sets a buy it now price of $100USD.
The starting bid is let's say $10USD.
Someone makes a bid for any amount less than $100USD and that's it, the buy it now price becomes unavailable from that point on (it would only stay if there is a reserve price too, and would dissapear in this case only when the reserve price is reached).
Meanwhile, more bidders arrive who maybe would have bought it at $100USD buy it now, however it's not available anymore.
They start outbidding each other gradually, and what do you know...the end price becomes higher than the original buy it now price. :-)
