Animal Crossing: New Horizons has been an absolute mega-hit for Nintendo thus far, bringing in millions upon millions of players. The game is so big that a mini-ecosystem has popped up around it, with people turning to the game to make some real-life money. Players have been hitting up auction sites, social media, and more to sell various in-game items and villagers. Not surprisingly, Nintendo is against this.
The Big N has actually spoken out about this phenomenon, and they want those involved to know that they should consider calling things off. Nintendo has stated that selling in-game items and islanders for real-life money is actually against the game's terms of service. Furthermore, Nintendo has said they'll consider going after each person/company doing this on a case-by-case basis.
We're yet to hear of anyone getting hit by Nintendo's ninjas over this matter, but it sounds like that could happen in the near future. If you've been making a quick buck off of in-game content, you might want to reconsider!