How are stores selling using copies of eye of the north?

on amazon, companies have used eye of the north games for only 10 dollars, but if the game is used, wouldnt that mean that the key has already been used and they are just selling you the disc and not the game?How are stores selling using copies of eye of the north?
Resale of software is legal as long as the person you buy it from is no longer using it.

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