Hallo.
For example, with Steam games it is so that you can only play it on your account. I would like to buy a used Xbox and the offer also includes FH4. Now I wonder if I can play that with my own Microsoft account at all? The way I see it, it shouldn't be possible due to the key, which can only be activated once for an account. Am I right?
Yes you are correct. If the previous owner has already used the code, you can only play through his account.
But the seller says that he bought it as a CD from Gamestop and that it is not linked to an account. Is that possible?
Oh yes, then of course. I thought it came with the console as code. You can play it and also resell or borrow.
A game on disk is never linked to an account. Only the starting score.