Well, it depends on the hat. Some old hats, in fact, should not be re-sold. These in particular are the one time event hats that are given to people that are quick enough to get them, such as revolution items and holiday events like the leporicon and the easter bunny. If these hats are re sold, it ruins the opporitunity and the challenge that players put in to get these hats, thus loosing the entire motivation for the event.