>>715304134
Every game cart comes with a unique serial number encrypted within it. When a game is dumped, the serial number comes with it. If Nintendo detects two consoles playing a game with the same serial number, it means one or more of them has pirated the game, and automatically bans all affected consoles from online services.
In OP's case the seller bought a MIG Switch, dumped all his games, and sold all the carts he thought he didn't need anymore. Nintendo's system detected the duplication and banned both, and the guy who actually owns the real games contacted Nintendo's support and got his console unbanned.