Blizzard has confirmed that earlier this week they fixed a major exploit bug that allowed Seasonal and Eternal characters to group up and trade. This has led to a major inflation of the trading markets as well as allowed Seasonal items to be used by Eternal characters.
Blizzard Global Community Director Adam "Pezradar" Fletcher confirmed this was fixed earlier this week in addition to bans/suspensions to users who fully abused this exploit.
Hi all -I just wanted to let everyone know that this was addressed a few days ago in our last hotfix and hasn’t been present in the game since. We have also actioned on select accounts related to this incident. Read more on wowhead.com