D4 has a Coop Mode, but you cant trade inside of your Coop group.
Blizzard says its because of botting or whatever. Okay.
Why not do it this way:
You can trade drops inside of your Coop group, BUT only to members of that group IF they were in that group prior to the drop of the item that wants to be traded.
This way bots cant trade their shit and people could still have fun playing Coop, trading with each other.