I only discovered the attitude thing by accident, when I was trading with someone towards the end of the game and accidentally gave them 1 gold too much in a trade.
This is also useful for improving non-trader attitudes to your character (eg if you get caught pickpocketing from them) - just click on "trade" button during dialogue, buy their crappy apple for 4 gold instead of 3 and hey, relations are hunky-dory again.
