As once said in the real swindle, if something sounds too good to be true, then it probably isn't the truth.
Personally, I don't see the site as really worth trying out, when you got greenmangaming, gooldoldgames, g2a and actual steam store. Not only they are 200% more credible, but they give you directly what you buy for low prices instead of putting in X amount of value and all you could be getting is an unfinished greenlighted Air Control or Day One Garry's Incident. for 20$. I do believe your attachment and do believe the site is not a scam, but regardless, I don't see point of using it.