I've heard that the auction house is available both via in-game menus (whether playing a game or not) and over Battle.net.
How can I access it?
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For anyone coming across this, the auction house (both gold and real money one) was removed from the game in 2014. https://us.battle.net/forums/en/d3/topic/9972208129
There's a fairly simple step-by-step guide on Blizzard's site:
View the Auction House FAQ for more.