Like most Magic: The Gathering cards, the value of Quiet Speculation 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 (Eternal Masters)
Its a ruinous gem indeed for the opponent, milling four cards on the end step of each turn in which you attacked
Top 10 Most Expensive Graded Pokemon Cards EVER
If an "Infernity" monster(s) and/or a Level 8 DARK Dragon Synchro Monster(s) you control would be destroyed by your opponent's card effect, you can banish 1 "Infernity" card from your GY instead
All Pokmon 4 have "4" at the end of their name in order to differentiate them from other Pokmon (curiously, the English cards still use to depict the 4)
I eventually found other Piplup collectors with their collections on Reddit and @citriina 's here on e4