Yes, the sites mentioned are professional Key-Reseller stores. Buying keys there is straightforward stupid.
The version I bought, is a DVD Box that was (now obviously) pre-owned by a single person and put in a "second hand" store. The shop was not that clear about it being "preowned" or "used" really. Yeah, I could have thought so.
Anyhow.
I am pretty sure, that I will get a refund from that store on eBay, I am not worried about that.
I am just p****d about the person, who is still using that key, because that is where the fraud has started.