The search for a is simple: Buy only from Rockstar, Steam, Epic, or authorized partners. While it is tempting to buy a $5 key from a random website, you are statistically likely to lose that money within three months when the key gets deactivated.
Once a GTA V key is activated on a Rockstar Social Club account, it is . You cannot transfer it, sell it, or give it to a friend. If you lose access to your Rockstar account, you lose access to the game. Treat your key and your account password with equal seriousness. Gta v activation key