Nintendo would stop used game sales if they could legally do it, but they can't
Legally they have the right to stop people pirating.
You can make a moral decision whether you will support developers or not by only buying new games, or buying second hand games or pirating. It's up to you.
Piracy is illegal, buying new or second hand games isn't.
They don't get paid on the second hand sale, but if someone can only afford to buy new games because they will be able to trade them & someone can only afford to buy second hand games then it's still 1 extra sale they wouldn't have had. That is the bigger picture.