Like most Pokmon cards, the value of Haunter depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Sword & Shield)
Fake cards often feature a lighter, washed-out, or purplish hue that stands out when compared to an authentic card
Stargaze allows you to dig incredibly deep into your deck, and because of this, it can shift the game in your favor
It's also worthwhile to note that Esper Sentinel doesn't just impact the first noncreature spell an opponent casts on their turn, but each turn
Mirror Wall
And then you have all the other factors of it triggers Greenshell, blocks with Reach, blinks well with Yorion, and is good to copy with Glasspool