There are already a couple of games working like this. So, probably not illegal. But as the method is becoming increasingly popular, it might come under more scrutiny.
English
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If you buy packages that give you random items and there's a possibility for you to not get anything worth that money, it's gambling and considered illegal to minors. Although there might be some conditions I'm not aware of.
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I think you can still refund the items if they are repeats.