Blizzard said they won’t be implementing an Auction House feature in Diablo 4.
In the latest Campfire Chat, Global Community Lead Adam Fletcher mentioned that there are currently no plans for an Auction House feature in Diablo 4.
Adam Fletcher: “Separately, this has been coming up a lot—auction house. There are currently no plans right now for something like that, but we hear you guys. We feedback things like that, let us know.”
If you remember back in Diablo 3, Blizzard had a Real Money Auction House (RMAH) that was eventually closed on March 18, 2014, because it negatively affected the core gameplay aspect of the game. Players were able to directly buy powerful gear with real money, creating a pay-to-win environment, where those willing to spend real money had significant leverage over those who did not.
This is probably something they would like to avoid in Diablo 4.