Not only could it be for hats that are about to be rotated out but "left over" holiday items that used to be available for purchase. You could include the quest items if you wanted, or just the ones you could pay for. Could be a smallish amount of items that gets rotated monthly so the inventory stays fairly fresh.

You could also make it a shop that players can trade in items for gralats like 10-30% of its original price like you would if you traded in a game to gamestop. Or trade it towards the purchase of another item to reduce its price more. For exp trade the pinata horse mount in and get 1000 off the price for a donkey mount. It wouldn't have to be an exact item type for an exact item type. i.e. you could trade in the valentines cat morph (3k i believe was the price for the morph) and get 750g off the price of a hat.