I'd advise not buying Windows from "someone".

Even if the key is "genuine" it could already be in use and marked as invalid by Microsoft.

You would then need to buy a new key.

It's easier to just buy it outright from a retailer.

Anyway if you don't heed my advice, when you boot up Windows type in the key. If it says you have an invalid key then you: a) purchased a pirated version of Windows; or b) someone sold you Windows with an invalid key. Windows is useless after 30 days without a valid key. After 30 days, if you still haven't purchased a valid product key, Windows will lock you out until a valid product key is provided.

Indeed, in the EULA for Microsoft products it clearly states the Original Purchaser of their license(s) may NOT resale the product. Even if the box is legit, MS may not re-activate to a new user.

Get it through your head. With Microsoft you pay for a license to use their product. Microsoft reserves the right to discontinue support without any further notice. You do not own the software.

If you like the idea of owning your own OS, download Ubuntu.

It should come with a sticker inside the box. If it doesn't come with a box then you can call up Microsoft to verify if the product key is legitimate. Besides, you would have difficulty activating the key if it was counterfeit.

dont buy used

go for a new one from the store

you might end up in loss if its already registered.

Just buy from the store...