I bought dead cells on goodoffer24 for less than half of current MSRP. They tout something called steam cloud activation. This is nothing but buzzwords. They simply purchase the key in markets like Asia or Russia etc. And then activate the product on your account using a sock5 proxy or a VPN. They require your account password, a SteamGuard token, and your account login. The process happens in literal seconds. Far too fast for a human to do, leading me to believe it's automated.
Pros - automated, fast
-much cheaper than g2a or steam
Cons- against steam Tos
-no way to verify that they delete your data
-have to turn over account login info.