Add inter-player trading
I think this has been discussed before but it isn't in the issue tracker yet.
This is on the roadmap (under "Trading").
Trading could be implemented a few ways.
One idea would be to have a trade window, add and remove items in the trade, and then once both sides have accepted the current terms (can include gold and items, maybe even other things in the future), the items are exchanged and the trade is finished.
Another way would be an auction system where players can auction items for a period of time and the highest bidder gets the item.
Yet another way would be a player market similar to GW2 with buy and sell orders that can be fulfilled by other players.
Marked draft for feedback.