It is not legal, but the advantage is that you don't have to deal with shady software like that pirate KMS server. Even though it is a breach of contract because it is not how these licences are supposed to be used, it is still recognized as genuine by Microsoft, so you get all the updates, etc...
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.
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.