I had the exact same problem as Merentha, except it happened in Worms.
When i actually got into the trade screen (second attempt, I was surprised that I was able to get in) the "Well-used polehammer" had no image associated with it (just a blank spot that acted like a weapon)
Hope this helps!
(Also, I had a problem with a weapon vanishing, I had filled my inventory with loot except for one slot, at the end, when the game calculates what gets what done to it, two of my weapons, a bow and a sword, were reduced in quality, both disappeared, and only the sword reappeared, in the one open slot in my inventory.. The bow hasn't shown up yet.. Bought a new one.. Actually, when i was going to the weapon shop in worms to buy a new one is when I discovered the above problem XD)