There's no way to sell a digital track secondhand, so yes, if it's a digital track on Amazon, it's officially licensed. Whatever publishing company Kim Hyun Joong is under has set it up so that his music can be sold there.

If this is the album you're talking about here, it says "Kim Hyung Joong by 김형중". The "by" doesn't mean that's the name of the person selling the album, that's just what the artist's publisher chose as his official artist name.