Blizzard recently turned off trading in Diablo 4 due to players using a new cheat to duplicate items. A message on the Blizzard forum from the community manager, filthierich, mentioned that the developers are actively addressing this problem:
We’ve suspended player trading in Diablo IV until further notice due to a duplication exploit. We are working on a fix to amend this issue and will update you once we’ve reinstated the ability to trade. We will continue to monitor this activity to ensure a healthy playing experience for all.
Any account that engages in gold and item duplication exploits will be actioned in accordance with our End User License Agreement.
We thank you for reporting this exploit to us and your patience.
Blizzard has turned off trading in Diablo 4 more than once. On August 15, they did it because players were using a trick to duplicate gold and items. They kept looking into the trading system even after releasing Patch 1.1.2. The trick involved a player starting a trade, adding items or gold, closing the game on purpose, and then logging back in.
Right before Blizzard’s crackdown on August, there was a jaw-dropping trade where a crossbow was sold for a whopping 30 billion gold.