So that is what happened... I was wondering why I recently got no cards after beating a game with a short first "episode". (In this case it didn't affect me since the second "episode" was an extra level section in the main game, and that section is much longer. (The first "episode" was basically a tutorial to the second "episode".))
If Valve is worried about buying a game, getting cards, refunding cards, change things so no game refunds after trading away or selling cards from that game.