here is the reply from steam.
ok so they are not an authorised reseller but they do not say that the key is invalid.
the keys could come off a retail copy .. who knows ?
Hello, Thank you for contacting Steam Support. This is not an authorized re-seller of Steam CD Keys. Please do not complete any purchases with this vendor. Only CD Keys included with retail copies or purchased from authorized retailers will be allowed. Let us know if you have any further questions or concerns.