Marco, please stop creating multiple threads on the same topic. Duplicate posts will be locked or removed. Continuing to do so may result in your profile being flagged or banned for spamming by the forum moderation tool.
Microsoft does not prevent you from purchasing a Windows license from a third-party seller, but it does not guarantee the legitimacy of licenses obtained that way. If you choose to buy from a third party, ensure the license is a Retail license and not a Volume license.