Is CJS CD Keys Legitimate Online Store?

Yes the keys are actually retail keys, not oem keys. So they are legitimate. They have many reviews on Trustpilot rated 4/5 with 65,000 reviews. Cjs cd keys do have a good reputation.

Microsoft selling the same OS for $285 and other websites selling it for $9, this is obviously a scam. The greedy software giant would never allow this.
This maybe an OEM key, which should not be sold and may only work for a short period of time.

They are likely reselling MSDN licenses. MSDN or TechNet Product keys are genuine Microsoft product keys, they are actually retail licenses, but it is intended for a particular product channel, either the Microsoft Software Developer Network (MSDN) or TechNet for IT Professionals who pay a subscription fee. But, the main purpose is for evaluation.
The great thing about licenses, unlike trial Microsoft software, MSDN or TechNet keys don't expire. Because the agreement under which the subscription is provided is a single license, none of the software should be distributed outside of it, and should only be used for one person and no one else. Unfortunately, regardless of the licensing terms, persons still abuse the program, they either giveaway product keys or resell it on auction sites. Microsoft licenses it in good faith that customers won't do so, but I guess human nature wins out.
But, once Microsoft detected that these keys are being abused, they will block them from further use, which will cause them to stop working. This why it is recommended that you purchase genuine licenses from the Microsoft Store.