If someone offers to sell you a designer handbag for $25 you can pretty much bet that you are dealing with the gray market or the black market. Same for bargain software keys. Computer manufacturers make deals with Microsoft allowing them to distribute copies of Microsoft software as a bundle with the computers they sell. They aren't allowed to resell the software without the computer. Sometimes employees or outsiders get access to the activation keys allocated to the company, and then re-sell them on warez sites. Microsoft also makes licenses available to vendors at cheaper rates in some countries, but the contract with Microsoft will forbid the resale in countries not covered by the agreement.
Laws for software licensing, copyright, and intellectual property differ widely from country to country. In the US this would almost certainly be a violation of the Digital Millennium Copyright Act and subject to criminal prosecution.
In most countries it would at least be a violation of the licensing terms and addressable by a civil suit by Microsoft.