You can buy legal keys from various vendors for roughly 15$ that are not obtained through illicit methods. The promos are usually advertised on techspot or guru3d. Anything below that I would be skeptical of.
The European Court of Justice states that a licensee has the right to sell a software license even if the conditions prohibit it. In other words. You Can Resell Your Software, Even If The Software Company Says You Can't
I have ordered them for other people I know " about 15 keys " and they all worked with out any issue at all .....
These may be old Windows 7/8.x keys you find going for cheap. Because you can use them to validate a Windows 10 install It also saves M$ the trouble to make and maintain new keys if they are already tied to an existing o.s.
How can you be sure they're not obtained illicitly? Most are stolen, reused or taken from MVS meaning they don't have any value, because they don't comply to Microsoft's EULA. Because Microsoft does not verify key usage. If you published a brand new retail key somewhere it would most likely get blocked in a few days, but such "careful distribution" rarely ever gets detected. Consider this: You buy a brand new box version of Windows 10 Pro. Let's say you sell the key 20 times. (a pretty safe amount) Now look at this: You spend $199.99 for the box. You sell the same key to 20 customers for (let's take @Solace50's example) $15 each. You spent $199.99, got $300 and you're left with $100.01. If you find cheaper keys, that means they're either stolen or a single key is being sold more than 20 times to different customers. Honestly, it's not that hard to grasp. - Reselling used licenses is legal (at least in Europe, don't know about America) but you have to keep in mind the seller must make sure they're not using them anymore, which they won't do, because it's just "bad for business" and no one can prove that they're still using them. You might as well use an activator at this point.