The "Hide Trader" button is on the bazaar shopping window. (/bazaar). It is on the far right, below the item display. You may need to either extend your window to the right, or update your UI (if using a custom one) to see the button.
Another, very necessary hotkey you'll want to make is an Assist key.
There are (at least) 2 that everyone should have.
One is simply /assist. This will assist whatever you have targetted, thus making its target yours.
The other is /assist [MAname] where [MAname] is the character name of the Main Assist (usually the tank of the group or raid, the group or raid will/should specify who this is). This will bypass any targetting issues that some may have trying to get an assist on. This will have to change depending on who the MA (shorthand for Main Assist) is.
Before you use these keys, make sure to type /assist off. This will turn off the autoattack-on-assist feature (and stores it in your ini file so you only have to do this once). This will help you avoid attacking too early (not a big issue at low levels, but as you get higher it'll be critical, good habit to get into early).
One last thing, remember that you have 10 pages of hotkey button spaces. This means that you can have a few extra hotkeys on a 2nd page, and swap them in and out as needed. (Hold down the hotkey to put it to your cursor, click into where you want to put it. If something is there already, that will be placed on cursor.)
<edit>Oops! double words are EVIL! I also need to learn not to forget words!</edit>
Edited, Wed Jul 21 14:11:47 2004 by cafeenoftheazurestorm