The best places are Hareruya 2 Akihabara (the worlds largest Pokmon TCG specialty store), Pokmon Center Mega Tokyo in Ikebukuro (official store with exclusive cards), Mandarake in Nakano Broadway (cheap singles and vintage finds), Card Secret in Ikebukuro (graded cards and serious collectors), and Pokmon Center Shibuya (Shibuya-exclusive merch)
There is a grading industry in the collectibles world whose purpose is to authenticate and determine the condition of a card, or a comic book, or a piece of sports memorabilia
Since these cards look so unusualespecially the fully blank onesthey tend to be some of the most sought-after misprints
It also comes back through the flashback keyword to give you extra value later on
Of course, many Pokedex entries allude to the fact that Phantumps are the souls of dead children possessing tree stumps, so maybe it has something to do with that