Purchase steam sale games outside of steam store?

You do not receive Steam sale discounts when buying a Steam key through a different platform. Different platforms may offer their own discounts though. As far as I know there are no official Steam key resellers. Some sites may be able to offer Steam keys via the developers or publisher of the game, through a bundle discount or negotiated price. Some "marketplace" type sites may offer Steam keys based on fencing stolen goods, or by reselling keys bought earlier for a discount for any reason. You may also be able to buy a game directly from a developer, or publisher, and receive a stand-alone or Steam version of a game that way.

In your case, it sounds like you authorized the payment through PayPal, and you must treat this situation as if the money is successfully paid. Steam may try to verify and complete the transaction when their systems are no longer under extreme load.

Steam will save unpaid "carts" of games for 72 hours when they are not paid, and will keep the discounts applied to them. When you open the Steam client you should see indicated that there are still items in your cart. You will also receive an email with that information when Steam detects that there was an error with a recently done purchase. When you receive this email it is safe to retry buying your cart with games.