PLEASE make sure you are buying a valid copy of windows 10 from a reputable retailer. To many websites sell fake keys that are not authorized in the U.S. and will not activate. Research the online retailer first on google/yelp etc BEFORE you buy a "Valid CD Key".
These type of sites like Ebay, Amazon etc have hundreds of people selling fake keys or keys that are meant for a different region but they pass them off as north american valid keys and they are 100% fake keys.
The only 2 places i would EVER buy a Windows 10 CD Key is from Microsoft.com or your local Microsoft store.
(and no, i do not work for microsoft, i just don't like seeing people getting ripped off of their hard earned money)
Be very careful on this.