Not all trades are through accounts or items. That's the problem. It is quite possible however, that there could be categories and an input field. Choose the category of type of item (Gaiaonline Account, Graphic, Neopoints, etc) and supply information in the fields. Both submit their trade and then the trade is up for moderation. MM's can have a control panel where they can access these trades and moderate them (check if things exist) and if they work, press the trade button.