As you know, a lot of sites are selling CD game keys but not all are honest and Ubisoft just triggered an action against them by banning lots of keys that have been acquired in a wrong way.
Last week, some gamers have seen games from Ubisoft disappear from their
Uplay libraries without any explanation. Watch Dogs, Far Cry 4, Assassins Creed: Unity, which are fairly recent have been removed since these were purchased on sale sites which are not approved by Ubisoft.
This has created a movement of gamers who have expressed their anger on
the official Ubisoft forum.
Indeed, if the publisher has a problem with key vendors, end-user has nothing to do with it, seeking just to pay the cheapest price possible. The only explanation given by Ubisoft is that the keys are no longer valid because they are wrongfully acquired and that gamers must contact the sellers to be reimbursed.
Note that if Ubisoft (as Steam already did) located its keys, the problem would not exist. But for a mistake on their side and sites that have taken advantage of the different currency rates, it is still the user who gets bullied.
Source:
Le Comptoir du Hardware.