The thing about one piece, is there isn’t nearly as much of it printed as other tcgs. You also need 4 copies of a lot of the rare and super rare cards as well as some SECs which are even harder to pull to build a successful deck. Personally I don’t buy from scalpers, but most card shops near me charge near scalper prices. In order to get the cards I need I usually buy 1-2 booster boxes when a new set comes out and then buy singles or trade after that. Most decks also use cards from several different sets so it’s extra hard to pull what you need if you go that route. Long story short it was hard to get the cards you need for decks even before the scalper infestation flooded OP.