During the Witches Festival for Elder Scrolls Online, you can collect items called Witches Festival Writs, quest items with specific requests related to crafting recipes. However, you may not always have the recipe needed to craft them, which prevents you from completing the quest. When this happens, you can be left with unfinished Writs, but you shouldn’t get rid of them, for they can be quite valuable.
There are a couple of things you can do with unfinished Writs. If you want to unlock the achievement and the witch hat item, you can store them in your house or keep them in your inventory for later. Even after the event is over, you can still make the recipes and turn them in as they still count towards the quest.
Instead of destroying or placing uncompleted Writs in your house, you could head to a Guild Trader. Guild Traders can be found in any major city, typically by an entrance gate. If you only need recipes to complete the Writ, you can buy from Guild Traders without joining a guild. However, if you want to sell items to a Guild Trader, you need to join a guild with access to a Guild Trader.
Once you are in a guild and find a Guild Trader, you can place the Writ up for sale. Many people may still be looking for Writs, and they can fetch quite a high price.