MarkoH01: I would never buy a game just to get some cards (in fact I never was interested in cards at all). I guess you are talking about Steam?
poor me, used to read op wrong :D Thought you are about steam library, since you giveaway steam keys.
As for cards - well, in past, before gaben killed trading with 2-weeks long holds and gem crafting, it was quite often situation when niche games (primary japanese) were on sale for less, than price of their cards. So it was profitable to farm cards and sell them. Now its extremely rare, and you only get profit if you do so from third party place (so - you dont need to pay for electricity, since possible income will be way less, than 5 hours of turned on pc). Rip.
As for gag - well, I have lots of hidden games there too. Mostly coz they was hella cheap (but, as revealed - looked good only on screenshots), lol