I got fed up with low level characters initiating trades without asking in Stormwind 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. This addons has been in private testing for two years. It is so simple that I didn't even need to modify it for TBC nor WoTLK. It still works.

1.5 has been optimized a lot before putting it up to Curse. Please try it out and comment!

Just a quick in-game use:

You can also enable or disable trades with a single bindable key or slash command.

/tc help - In-game help
/tc - Enable / disable trades manually
/tc status - Show trading status
/tc default on - Enable trades by default
/tc default off - Disable trades by default
/tc ex add <zone> - Keep trades on in <zone>
/tc ex remove <#> - Remove <list number> from exception list
/tc ex list - List exceptions

Default setting affects addon functionality only in cities.
So by setting default to on enables trades in cities and makes addon usage obsolete.
You can use exceptions instead. For example enable trades in Shattrath and Dalaran but keep the rest of the cities blocked.