The term “illegal” here is more towards the breach of contract (ToS/EULA) of the game in question.
Yes, this is not always covered in their EULA/ToS in every game sold in itemku. But there’s a pretty decent amount of game that do have this EULA/ToS. Sometimes, it’s not an EULA that covers one game only, there’s some developers that choose to use one EULA for all of their game and you can check those in their website. Most mobile game developers have these.
Note: TechInAsia published an article regarding this too. They also have a lawyer opinion there, so do check it out. This is the first time i stumble upon it too. https://medium.com/@unjukin/virtual-gaming-items-marketplace-itemku-raises-1-2m-but-legal-issues-persist-26c66c401ef5