Badges are like Playstation's Trophies or Microsoft's Achievements. Some of them (actually most of them) are earned by playing games and doing different tasks (again, like Trophies/Achievements) but others can be earned with those cards.

Some games give you cards and once you have the full set of cards, you can forge a badge. The same is true with the cards of this Summer Sale, once you get the 10 cards you will be able to forge the "Steam Summer Adventure 2014" badge. Of course, with each of those badges, you get Steam points that make you go up in the rank.

That's why people pay for those cards, to get badges to get a higher rank.

Interesting, so basically you need to collect all badges associated with a game for it to be useful. Well then I probably should sell all my "Steam Summer Adventure 2014" bc I only have 2/10

Seriously, who voted for Casual - Simulation? I totally needed KoF. Ugh.

New sales up...
The Stanley Parable - 60% off, $6
Skyrim - 75% off, $5
...
"steam store experiencing some heavy load right now" LOL

That's exactly what I did two years ago

Updated with today's deals. Skyrim price is great but i just don't have 100 hours to spare. Enjoy guys. :)

The team thing is ingenious way to make people spend so much money. Obviously, green team represent. 

 Charged up today.

I would wait a bit longer. Every three votes you'll get a new card and joining a team grants you another one. Add the ones you can get buying games and it's not impossible to get the badge.

Of course, it's not that easy as the cards you get are random and sooner or later you'll get one that you already have. And then is where trading cards or selling them comes into play. You can trade your spare card with a card you don't have with one of your friends (I think you must be friends).

In any case, unless you are 100% sure that you don't want or don't have a chance of getting the badge, I recomend you to wait until the last couple of days to decide what to do with the cards.

@green_sky: It's interesting that  the purple team is always up there but the other one changes, yesterday it was the red team and today is the yellow one. Meanwhile, my poor pink team...

£2.49 for skyrim thanks

$1.86 for half life 2...

yes laugh at me but it's the best I've seen that my pc can handle

Since both games use the same engine, you could try to get Portal when it reaches 75% off (it has gone that low for the same sales so I don't see why this time would be different). But only if you like Puzzle games, ofc.