It probably opens some potential exploit for quick leveling or some gold transfer or even real currency trading.
At least if you will be able to set price.
Not sure if it is a problem, but at this moment game obviously restricts most of between players transfers (only exclusion is hiring hero, but this is for fixed price and only once per day per player, so minimal).
Another one is "cut-the-line", but rewards are so ridiculously low, that I am not sure any guild uses it.
And I am not sure it will be popular: quite obvious that there are high demand items (nobody will sell them) and low demand items (nobody will buy them). For higher levels who may have some excess of mid level items gold does not matter anymore.
In general I support more in-guild interaction but mechanism needs to be thought.