He is indeed just doing "mesugaki" edits of free cards and putting them for selling on his fanbox, I've notice him ripping off gin26, leona_skyrim, SAM and some smaller free card creators a very long time ago.
I don't really wanna spam in this thread with this matter, but here's a quick exemple:
The card you see on the thumbnail of this post
https://www.pixiv.net/artworks/124693635 is a very cheap and shameless edit of this free card from Manjo (a free card creator)
https://www.pixiv.net/artworks/122972140 if you look closely you will notice it's basically same card with black hair and different eyeline and iris, very cheap and lazy, right? Manjo even commented on his post.
Another one:
See this card here
https://www.pixiv.net/artworks/112263318 doesn't it looks familiar
https://www.pixiv.net/artworks/97543295 ? Oh and let's take a look at one of his newest "creations"
https://www.pixiv.net/artworks/130429085 Same card with different skin and hair color. Wow! Reselling the same card twice? Incredible!
Some people are really shameless and dirty, these practices are likely to kill many real creators who distribute cards for free. These people are the poison of this community and the reason creators like hakkadrill, en, tak and others started deleting their posts. But even so, SingerYue is a small fish and he is barely active. People like NETO and NHM-Cards do much more damage to the community with their mass reselling spam, they are the worst.