Question about rarity for gengar figures on Y!J!

Hello all! I recently have been watching a set of gengar figures on yahoo japan slowly go up and up in price and I am amazed at what the price is out now. I was wondering could someone with more gengar knowledge than I clearly have explain to me why these figure or which in particular are making the auction reach the current price?! Thanks :)! I added a link.

http://buyee.jp/item/yahoo/auction/m163316885