The process is you login to the Microsoft site with your MS account and enter the product key. MS then works out what exact version the key applies to and asks you what language etc.
It then starts a download of the installer which runs in the background (you can keep an eye on progress via the system tray).
On first running it will ask you for the product key again so it can activate and whether you want open docs or MS formats as the default.
All the sellers say you must use the key within 24 hours and you must follow the process i.e. don’t try and use it on your own installer. Why I don’t know. They are clearly sailing close to the wind but as Microsoft check the key and provide the installer (I think it may be customised in some way) and link the key to your MS account it must be legitimate if with restrictions.
Microsoft do all sorts of licences for the same product. Indeed in the Corporate world we found we sometimes had to buy 2 licences. If you had a virtual machine running on a server that needed an Office licence. OK, fair enough, but so did the PC accessing that virtual machine!
EDIT just been talking to someone I provided a key and instructions for a while ago. He waited a week or two before following the process and it worked just fine.
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