For high-value cards, buy already-slabbed copies from PSA, BGS, or CGC where the card has been professionally authenticated
As far as strategy goes it is very important to move Darkrai from your hand to your bench when you need the have a possibility of the defending Pokemon not attacking, also I would definitely play Dark Slumber right away to make the defending Pokemon asleep and then use Dark Resolve especially if the defending Pokemon is asleep and it should be because of Dark Slumber
Basic Grass Energy #1 (Pokemon Scarlet & Violet Energy | Pokemon Cards) prices are based on the historic sales
If you ever played similar games, this game will feel very good from the start
No, Stifle cant target turning the disguised creature face-up, as this isnt an activated ability or triggered ability
Please note that there can be a premium to pay at these types of specialty stores like Card Kingdom though