I buy way too many games on Steam just for the trading cards. After all the games' cards drop, I will usually buy/trade for more cards until I have enough to make a badge. If I ever get a booster pack for that game later on, I will complete the set again unless that badge is already max level, in which case I will sell the booster pack.
I've actually made a bit of money here and there by doing this - you just have to be careful what you buy and how you sell it, of course - but I also know that I'm supporting a lot of crappy shovelware devs by not only buying their awful games but giving them a little money after that (devs get a cut of the price of every card sold), and that I'm doing it just so that my next big Steam purchase will hurt my wallet just a tiny bit less.
There's a cycle of addiction at play here, of course. Every badge you create or upgrade gives you XP, and every ten levels you get a better chance to receive booster packs, which are created when someone else out there makes a badge. I'm well over level 100 right now, and am starting to receive boosters fairly frequently. Sometimes I just don't want to mess with them and quickly sell every booster I have, and I realize that I make the most money when I'm the most apathetic towards the whole trading card thing. And yet I continue with it, in hopes of making that $29.99 game a $28.71 game one day.