A game key is only useful if it activates on your account, in your region, for the edition you wanted. Before you buy anywhere, check four things.
Region: keys can be EU-only, region-free (global), or locked to a country. Platform: Steam, Epic, GOG, and console keys are not interchangeable. Edition: standard, deluxe, and ultimate include different content. Protection: buy where a wrong or used key is refunded.
On KeyVault every order includes purchase protection — if a key does not work, you are refunded to your original payment method, with a 24-hour response window.