It is a vintage TYPE fabric as the seller said....just like reproduction type fabrics. It was a very popular fabric many, many years ago. Can't remember when but do remember those mushrooms. People bidding must remember it from their youth and want to create a memory that made them happy.
Beauty is in the eyes of the beholder. We all have our likes and dislikes. We all pay more or less for something we really want. So it is really not surprising to me that there are people willing to pay for that fabric. Just like anything else if someone wants something bad enough and they haven't seen or found it and now have to chance to get it they will pay whatever they want.