The problem occurs even without bartering. I had the exact same problem the first time I tried to trade with the spell vendor on the market place. She would tell me every offer I made insulted her and increase prices every time, even if I offered more than what she asked for. I never took barter, in fact I never take barter.
Stabbey is pointing out something worth looking into, as I realized after trying so many times to deal with the said vendor that her attitude towards me was -50. I couldn't be sure the attitude ajustment wasn't triggered by so many "insulting offers", I hadn't checked before. I hadn't talked to her prior to trading, and most certainly never even thought of stealing from her.
The issue kind of solved itself after awhile. I mostly gave up on trading with her, went on to do quests and came back some hours later in the playthrough thinking maybe if I talked to her with Madora she's be more willing to trade. She cared little about dealing with lackeys, but her attitude towards me had gone back up to -10 and I was able to trade normally with her from that point.
