My trader has also reset. Used one of the followers to level up attitude instead of my character. Someone on steam suggested it may be due to switching out party members. It seems to be the cause from the test I just did.
That would be correct. Vendor attitude is specific to each individual character. Prices are also effected even more strongly by charisma score than by their attitude towards you. My CHA 12 barbarian gets more or less the same prices with maxed out rep as my bard did by default when her CHA was 16. So if you play multiplayer, make the face also the shopper, and only bother to raise vendor rep on them. And even in single player, maybe make whatever character in your group has the highest charisma score your shopper.
