I think he means, Kinguin is like a marketplace for other companies to sell you their keys. There will be bad companies on there, that is what rating systems are for (hopefully).
I've bought multiple keys from them, one was a problem, they processed a refund and I bought it from a different reseller. It was the reseller who gave me the bad key, not Kinguin.
So list the bad reseller and don't use that company again.
Now Kinguin process payments, if they either refused to help you out getting a refund(which I believe they do themselves then work it out with the reseller) or if you get credit card details stolen or something by Kinguin as they take the payment and then pay the reseller, then you blame Kinguin.
I can't remember the name of the reseller who gave me a faulty key now, it's been a while. Worth remembering I used the e-mail/ticket system and didn't get a reply quickly, went to the online chat which I didn't see at first and got through straight away and they got onto the problem.
EDIT:- I think you edited in the reseller bit after I started typing a reply. The payment should be to Kinguin then they deal with the reseller. I haven't noticed Kinguin listed as the seller of keys on any titles I've looked at but I suppose that is definitely possible.