G2A, g2play and kinguin are grey market sellers; the people selling keys there sometimes get them via illegitimate or fraudulent means. Some companies deactivate those keys.
http://www.gamespot.com/articles/ubi...a/1100-6424917
One of the ways these people get the keys is via fake game key requests:
https://www.reddit.com/r/gamedev/com...tips_to_avoid/
Or even giveaway requests, only to sell them all:
https://www.reddit.com/r/gamedev/com..._selling_them/
Most of the time it is fine however.