The original real-money Auction House was permanently shut down years ago. Today, items of Legendary or Set quality on PC are strictly upon dropping.
On non-seasonal console profiles, restrictions are looser. However, on PC and in all Seasonal modes, trading is highly locked down. How Third-Party "Buying" Actually Works buy d3 items
You join a group of professional runners. They farm high-tier Greater Rifts with you and drop all shareable Legendaries and Set pieces they loot directly to your character. The original real-money Auction House was permanently shut