As others have mentioned there is an Auction House in the Battle for Azeroth city hubs, if you’re an Engineer 
That was true back in Legion as well, during that time the AH in Dalaran in the Broken Isles was just for Engineers, it’s a perk for being that profession, your own little robo buddy Auctioneer.
It was opened up to others when Battle for Azeroth released to make it a little easier for people still leveling up in that content.
Warlords of Draenor you could build an AH in your Garrison.
Mists of Pandaria the Shrines each had an AH again for Engineers only back then and later opened up to everyone.
Dalaran in Northrend too back in the day, during Wrath of the Lich King it was Engineer only, and opened up to others afterwards.
I don’t actually remember if there was one in Shattrath… it’s late and nearly my weekend though 