I think I saw Kinguin as a sponsor on Linus Tech Tips so I don't know. $35 seems kind of legit. Once I bought a Win 10 license on eBay for $10 and eight months later it was revoked, lol.
The problem I have with key resellers is I don't know how the reseller obtained the key. The only few ways I know that you can legitimately obtain a Microsoft product license key for less than retail cost is by joining one of Microsoft's programs. And of course, Microsoft simply doesn't let anyone join said programs and the distribution of licenses is limited to just that person. So I believe the most common way is the person stole access to a payment method and started milking it for all its worth. Games I'm a little less skeptical of considering there's plenty of options of "here's a bunch of keys with your purchase", but I still cannot in good faith partake in key resellers.
There are plenty of free (as in beer and/or freedom) alternatives to Microsoft's products, with Microsoft themselves providing free (though limited) versions of Word, Excel, and PowerPoint. And again, you can use Windows 10 indefinitely without a license.