Hello,
If they are significantly cheaper, chance is - they are not legitimate.
Microsoft doesn't prevent you from getting your license from third party, but Microsoft also doesn't guarantee that you would get a legitimate license if you decide to buy from third party.
Microsoft has some challenges when it comes to controlling websites that share pirated content. They don't have the authority to act outside their legal boundaries and there are too many websites to monitor effectively. It's also technically difficult to spot pirated content correctly, and it would require a lot of resources. Additionally, it's mainly the government's job to enforce copyright laws. Concerns about privacy and inappropriate censorship also make it difficult for Microsoft to take direct action. Instead, they focus on supporting laws against piracy and educating people about it.
If you fall victim of pirated software, you can report them here: https://www.microsoft.com/en-us/howtotell/cfr/r...
The reason many third party can sell them cheap is because they resell volume license. Volume licenses are meant for organization users and they come with certain limitations - such as limit on how many times you can activate, and the requirement to connect to organization server to renew license so it will not expire, etc. They are meant for big companies where they buy license in bulk, hence cheaper. But if they are being resold to home users, it is considered a violation of licensing terms. They may seem to work at first, but when it comes to renewal time or when it exceeds the activation limit, your license can all in a sudden stop working. We see this type of issue way too often in this forum when people buy licenses from third party, hence I caution you.