Hey there,
I also have the same issue!
Strangely enough, this happened after the arena battle and only if I control more than 2 characters.
If a friend joins and takes some characters off me, Arran will happily sell me his wares.
However if controlling all 4 his conversation will end before it has begun.
Not even frantic clicking at the trade button as it shortly appears opens the trade window.
Even weirder, when I offloaded characters to a friend and he left a bit later, there was a "ghost player" in control of these characters. I could only reclaim 2 characters, and the ghost player kept 1. Saving in this state creaated a corrupted savefile. Luckily I had a backup!
