Auction Art

How auctions work of contemporary art?
I was looking at this site: http://www.saatchi-gallery.co.uk/blogon/art_news/this_weeks_news_round-up/5746 Contemporary art sold for over £ 80,000 each! Who decides which artists are allowed to work outside the auction? How you decide? Why sell for so much?
There is a great consensus about what is valuable art and what art is not. Mainly the reason is that work that artist formerly sold for a high price. Everything is madness.
Sotheby’s contemporary art auction