The video attached to this post showcases the current state of the new WIP trading UI. As seen here, the way it works is you individually indicate items for trade, and to perform the trade you'll press the "Trade" button in the middle. With this new UI, you'll be able to rely on the game's UI to help you calculate what exactly you need to trade for something before actually doing it. In other words, it will feel much more like actually trading.
Merchant weight capacity is currently planned to be handled a bit differently — merchants won't actually keep items in containers like a player anymore, for simplicity. As a result, the additional weight capacity that a container would provide is added to the merchant's weight limit.
If you have any feedback on what I have so far, feel free to share it.
And my question is — what should I do with the existing trade actions?
IE, right clicking on an item, the "trade for X credit" actions. For simplicity I'd like to remove them, keeping all trading in the UI, but if people regularly make use of them I can keep support for them. What do you think?