Might be wise to check the updated Auction House FAQ to find out exactly what is happening. Or watch the video below.
Also, here’s a quote from Blizzard on why they decided to close it:
When we initially designed and implemented the auction houses, the driving goal was to provide a convenient and secure system for trades. But as we’ve mentioned on different occasions, it became increasingly clear that despite the benefits of the AH system and the fact that many players around the world use it, it ultimately undermines Diablo’s core game play: kill monsters to get cool loot. With that in mind, we want to let everyone know that we’ve decided to remove the gold and real-money auction house system from Diablo III.