I have been running into this issue a lot on my current playthrough.
Both Anna (I guess that was her name, the girl from the Fabulous Five inside the Tavern) and Esmeralda refused to trade certain items, mostly magical weapons/armor.
After reading the previous post I had a little test:
I wanted to buy a magical(green) 1-handed sword (level 7 or 8 I think) from Esmeralda, it was listed initially as ~187 gold.
Of course she refused to trade it for that price so I put in more and more gold until she finally accepted my offer. I had to put over 1400 gold into the trade window for her to accept, keep in mind the initial listed(!) price of 187, that's almost 10 times as much.
So I guess it is indeed an error with the displayed price of certain items (scale button also only adds gold until listed price is reached)!
