Buying a CD key from a CD key marketplace is always a risk since the site themselves usually isn't the one selling the keys but an individual seller using the site.
Just like with Amazon. If you buy something on Amazon, it's usually not Amazon you're buying the product from, it's someone else that is using Amazon as a marketplace to sell their product.
I would mostly avoid the site Gamers-Outlet
But use the site Allkeyshop to browse the best prices across CD key sites.
The US version of the allkey site is called cheapdigitaldownload.
Also as mentioned, you can get great deals a lot of times on fanatical, there is also humble bundle.
Chrono.gg is cool too, but that one is only a single game ever 24 hours.
If looking for free games...