A friend is trying to buy a game and found that www.empiregaming.com has the cheapest digital download he can find. It has a high TrustPilot review rating, however googling TrustPilot results in multiple articles about TrustPilot having a lot of fake reviews.
Aaaand their website is 404. They're likely operating like other cheap grey market resellers use stolen credit cards to purchase game/software keys and resell for a low price.
With Windows selling that low it's likely to be the same as what you find on eBay for less - Volume License/MSDN keys which are often assigned to Businesses/Developers with mass activations. E.g. Win10 Pro with 100 activations per key. They're not legit and sometimes get pulled by Microsoft.
Or they get the keys in brazil/russia etc for cheap form wholesalers or microsoft. I just got windows 10 pro from them I will let you know if it gets canceled but it activated fine.
The windows licences for 25quid suggest they aren't 100% careful that they only sell legit software.