I don't think it is illegal on the part of the buyer, who would be a victim in this situation.

But just like buying counterfeit goods, it is not clear to me, I mean if you buy your key from AliExpress, isn't it like importing counterfeits from China? That's something one should ask a lawyer. Anyways, these keys can be revoked any moment (and it has happened to me, with an Office key). If I had a business, I would avoid them for that reason alone.

Well now that you have read this forum thread and responded to it that’s proof that you knew that was a possibility at least. You can’t claim “lack of knowledge” if you do it and Microsoft comes for you.

But I am not a lawyer, perhaps on this case that doesn’t matter.