Tradecycle is a light-weight and easy to use addon for automatically disabling trades when your character enters a city. Yeah, I know. Being rich is a bitch. :)

Some history for the addon:
I got fed up with low level characters initiating trades without asking so I disabled trades alltogether. Then I got tired on always having to go to interface options panel just to set it back. First I made a macro switcher for it but I wanted more. I wanted it to be automatic. An addon which automatically disables trades when I enter a city. And here we are now.

Just a quick in-game use:
/tc - Enable / disable trades manually