Anyway, I'm not sure if anyone really buys Windows at retail prices. Enterprise (subscription) and OEM costs are much lower, and for personal use you can buy OEM keys off eBay or AliExpress for a few bucks.
The opaqueness of Windows licensing for "pro" devices makes us assume that there are heavy discounts, which massively depends on the manufacturer- framework could not be at a size that gives them preferential pricing (even if we assume that licensing is cheaper for other companies).
I can say that it is more ordinary for end users to get discounts rather than OEMs, unless there is a lot of negotiation (including bundling crapware, as we well know) in the tech industry when it comes to bulk licensing.
Is it legal? If it's not, isn't it easier to just use crack? E.g. pirate KMS server.
You may get your license revoked it the seller get caught and as a result, you may need to get a new one. As an individual, you are extremely unlikely to get into more trouble (same thing as with cracked software), but I wouldn't use these licenses as a business.
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.
But I am not a lawyer, perhaps on this case that doesn’t matter.
Microsoft of course knows it, they are silent about the issue, preferring fear and doubt, just like you demonstrated.
MSDN subscribers can generate an unlimited number of keys at no cost, but they are for use by the subscriber only. Selling these keys is an abuse of the program. There is no resale here and you don't really get a license, you get a key.
So while some might come from MSDN subscribers, they often come from unused enterprise ones. The MSDN sellers are creating a big risk to themselves.
I would look whether they are located in EU (due to the ECJ ruling) and whether they also sell something other than what is available over MSDN.
A lot of people in fact point to this ruling as a reason why more and more companies switched from perpetual licenses to subscription licenses. Without a full licenses there is nothing to resell and this ECJ ruling doesn't apply