Blizzard has disabled item trading in Diablo 4 after a duplication glitch was spotted by players. The duplication glitch is exclusive to the Season 2 content at the moment and reportedly revolves around killing the Echo of Duriel over and over again to obtain Uber Unique items. While it is easy to summon the Echo of Duriel to kill the demon, the items required to do so are not so easily obtainable. However, players found a way to copy these materials
On the official forum, Blizzard says item trading has been disabled. Likely to prevent players from copying the materials used to summon the Echo of Duriel.
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 usually doesn’t take too long to sort these issues out. You’ll likely be able to trade again within the next few days (or hours). Back in August, Blizzard also disabled item trading when players discovered an exploit to trade items and gold. The feature was turned on again a few days later.
Source: Blizzard forum