They’re purchased with stolen credit cards.

So then you buy it cheap and activate it on your Switch, then the person who got the credit card stolen notices the fraudulent charges on their card, and reports them to their bank.

Then the bank issues a chargeback, so Nintendo doesn’t get paid for the code. Once Nintendo doesn’t get paid for the code, Nintendo invalidates the code, and bans the account that redeemed the code, because as far as Nintendo knows, it could have been you that bought them with an illegal credit card.

Game codes that are priced too good to be true, usually are.

If a game has never been on sale for cheaper than the code you’re buying, that means the person paid for it with a stolen credit card. That’s why they don’t mind selling it for less than it cost to buy it.

Do not buy keys from key resellers. You will get banned. The positive reviews you saw probably just hasn’t gotten banned yet because the person who’s credit card got stolen hadn’t noticed the fraudulent charges yet.

Don’t do it.