So I am a little late in my reply as it didnt notify me someone responded... It is NOT legal to install windows on a pre-owned or refurbished machine with anything other than a Microsoft Refurbished COA which comes with a refurbished product key for the end user. Using the OEM media is NOT a legal method per microsoft. However to create a product key using a refurbisher COA, you need the OEM COA serial number. There are not stickers present and microsoft says this means you can't refurbish a Windows 8/8.1/10 PC legally... This is why I came here to find out what refurbishers who follow the rules have found.
Using a OEM disc is not legal. If you dont believe me, ask Microsoft.