If you start a trade with a trader who wanders around and they move too far away, the trade screen remains open but becomes unusable. The most irritating example is Derryth - after you get her the Noblestalk you cannot engage her in conversation and she wanders around all the time, so it is difficult to trade with her, which effectively takes a trader out of the game.
Trading with a character either needs to lock their movement or the trade screen needs to close when they move too far away. I prefer the first - that would allow completion of trades with several wandering characters much easier.
