It is not that trading without bots is possible it is that Blizzard wants to avoid Play 2 Win - apparently at any cost. So, until you have a system that in no way can involve money at any point, there will not be an auction house, there will not be trading. Additionally, the first time Blizzard hears about someone making money - just like in D3 - they will investigate and shut everything down.
Even if you add a fee; even if you add a short time limit; even if you come up with a new currency the second there is real world money involved Blizzard will shut it down. They may even shut it down if someone is caught trying to farm the new currency.
The problem is that someone, somewhere in the name of free enterprise will try to figure out a way to make real world money. If you do not instantly ban anyone doing this, Blizzard will shut it down. If it is too much trouble to enforce (think about bots already in game), Blizzard will shut it down.