Ebay Buy it Now: I thought that option went away after a bid?

What’s changed? Sometimes I see or post auctions that have a buyitnow and the buyitnow option disappears after a bid…on others it remains.

What gives?

Buying with the Buy It Now option

Is there any way to know which is which? That seems rather strange…

I’ve seen that on auctions where there are more than one of the same item for sale and you’re just bidding for one.

Might it also apply to items with a reserve (i.e. minimum price that’s higher than the starting price)? Or is that possible with Buy It Now?