Yes, Kinguin and G2A are “peer-to-peer” places, pretty much like the ebays of key sellers. They don’t really source keys themselves, but implement a marketplace of various individual resellers. This is why the fraud danger is much higher.
CDKeys is more akin to a Greenman, where you’re not buying from various individuals but from one retailer that has sourced keys themselves. It’s also why getting refunds etc. is much easier.