Kinguin it's like G2A. Grey. The problem it's this. Anyone around there can sell keys, but most places on where you can buy keys you are not allowed to resell them. Those people that sell keys there are not official sellers, are 3rd party individuals/companies that resell keys.
It's not as safe as you would buy from official sellers, as you don't know from where those keys came and how where they bought. You might end up with your games removed from your account, as the keys were bought fraudulent, with stolen credit cards. It does not worth the risk, if you ask me.
There's region restriction too, some keys might not work in your region. Better stick to official sellers, even if you pay a little bit more. If you see a game at huge discount on those sites, i don't know, it's $20 on Steam and they are selling it with $2, search for the promotion/bundle, as there is sold somewhere at a huge discount or as a part of a bundle, and take it from there.