The triggered ability can target any creature, including one that is already tapped. In that case, the creature stays tapped and won't untap during its controller's untap step.

The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.

If another player gains control of Merchant Raiders, the creature will no longer be affected by the ability stopping it from untapping, even if you later regain control of Merchant Raiders.