Yea, you agree on a post who is going to buy what and that.
You can request a mediator (a third party individual from the website, who is like a middle man for the trade)
You discuss the trade here, and through PMs etc, you give each other you in game names, and only talking about the trade is done through this site, do not mention the trade in game.
Also you have to actually trade the gold, if you mail it, blizzard has more ability to track the gold and then you can run into issues.
and taken from the moderator's rule book on WoW trading
"Trading WoW Gold:
You must be in possession of the gold you are trading before you post a thread or response.
You must state how much gold you are selling and which realm the gold is currently on. Unspecified or "unlimited" offers are not allowed.
You are not permitted to take orders for gold, or attempt to trade gold you do not have.
Gold may not be traded through the mailing system. All trades need to be done "face-to-face" through the in game trading window.
Illegal Trades:
These are the things you may not trade for.
WoW time cards. These are only permitted in Real Life Trades with access through the Donor Forum. Please read here for more information.
No selling of accounts (Battle.net OR World of Warcraft), character names, prepaid time cards, prepaid character transfer cards, or any transactions involving money (you also may not trade in-game gold/items/services/etc. in exchange for any of the previously mentioned items).
Beta stage game invitations or other miscellaneous invitations
Scrolls of resurrection
Account authenticators
TCG codes
RAF
WoW Services:
With the addition of the My Events system, we are now allowing all users to host and request service type events. You can use the My Events system to post your services or to request them, including leveling, honor jobs, professions, etc. If something you are wanting is not in the suggested result list, you can edit the event description accordingly!
Services may now also be posted in the trading forums when using the "New Service" button to create a thread.
"