Don't PM users who purchased your items, in an attempt to get them back when you mis-price the item. All sales are final so when you are selling your item you need to check the prices. If you mis-price an item, please do not PM the user asking or demanding for the item back. They are not required to return the item to you and many buyers consider this rude.
All sales are final. There is a confirmation screen so that you can check and make sure all information is correct. If you sell an item for less than the current going price there is nothing you can do.