TCGplayer is the undisputed king of the secondary card market. If your kid is serious about Pokémon, Magic: The Gathering, or Yu-Gi-Oh, this site is inevitable. It’s not a 'kids' site'—it’s a massive marketplace that requires an adult in the room to navigate the logistics of shipping, condition grading, and actual currency.
It’s an excellent way to teach a kid about supply, demand, and the value of a dollar, provided you don't let them treat your bank account like an infinite mana pool. Stick to the 'Direct' program to avoid shipping headaches, and use it as a tool to move them away from the 'loot box' mechanics of buying random packs.

