Would it be too difficult to have a button from the adventure so that you could post loot spots onto trade? This would be easier and instead of working out what a fish is being sold in lieu of loot spot-wise, you could see it plainly. Players would still be able to do it the current way if they preferred but it might be a better system to implement.
What I envisage is a button on the adv menu that says 'Invite via Trade Office' or something and then when you clicked it, it would create the 'offered' side in the trade office and the player could then choose the asking price and voila - it would be posted.