There is resale value in them - people are often looking for better quality stickers for playsets when they have them missing. But its not particularly large value unfortunately. However if you were selling, if they are in nice condition and easily reattached, I am sure you will find a new home for them.
However, if its for a Dream Castle you are keeping and will have on display then I would simply use some blu-tac [I think it's called mounting putty in the US] to put them back up. This way you don't have any glue seepage/damage issues to worry about and you can even take them back down in the future if you wished. If you are selling the Dream Castle I would simply include them with the item so that the next owner can reattach them as they wish.