In classic the amount of items that transform your character are very few and the majority of them last about 5 minutes (good one blizzard). Either that or they are zone restricted or they must be equipped.
This makes items such as the Savory Deviate Delight Savory Deviate Delight - Spell - World of Warcraft a very popular item.
The Savory Deviate Delight is easy to acquire so it doesn't sell for much, but, makes up for it in sales due to it's popularity.
The Orb of Deception is also a popular transformation item, however it's a trinket so it must be equipped and you guessed it, only lasts 5 minutes.
The Hallowed Wand Hallowed Wand - Random - Item - World of Warcraft on the other hand, not only lasts 1 hour, but also, transforms you into the same things as the Savory Deviate Delight and some others like Leper Gnome, Skeleton (like the Noggenfogger Elixir) and a Ghost. Which don't break on combat making them much more desirable. And as you may have noticed you don't need to fish or farm a recipe for this wands. All you need is to visit an innkeeper and select the "Trick or Treat" option and get a little lucky (you don't always get the wands).
So the idea is, since you can't use the item on yourself (can only use it on party members) you could sell this transformations and seeing as you can only obtain this item along with it's unique (to my knowledge) transformations during the Halloween event it could potentially sell for more than any other transformation item (keeping in mind it's advantages, like not breaking on combat).
I should add that they are soulbound, so if you are going to sell it, you have to use the character you got it on which can be annoying. Also idk about other people but I've only received the random wand so far so that means you do not know what the person will get so it could be a bad one like a wisp which can't be used in combat.