What to do with tradin cards?

Sell it for cash or just buy games which ever you find first.

Trade it for games (some people accept that) or sell them, and use up the wallet funds to buy games - in average without foil cards the 40 cards you have roughly can have the value of 1.5-2.5€ (if you can sell them)
Don't exchange them for gems, the conversion rate is really bad.

If you don't care for crafting, sell them.
If you care for crafting, wait till summer sale to craft, probably there's an event of some sort.
Also you can trade with them.

Trade them for games , or sell them on the market .

sell em all

Stuff them up your anus for maximal pleasure. Jetzt muss ich gehen.

I usually just sell them on the market. It doesn't sound like you'll get much, but it all adds up. Although, if it's the really cheap cards, sometimes I'll keep them and make a badge.

If this years Summer Sale event will be the same as last year, keep your trading cards until the event starts. Then sell them at higher than average prices.

Craft them or sell them during Steam sale for higher price

Du kannst diese karten schon in der marktplatz verkaufen um geld wie Steam Credit zu verdienen!
Es tut mir leid, ich kann nur ein wenig Deutsch sprechen! :D
Edit: Wenn du schon geld verdienen hat, kannst du spielen am Steam kaufen, aber dafur musst du VIEL geld verdienen. Manchmals ist es nur 0,8 cents pro karte

Im Notfall handle sie mit mir, habe noch ein paar Keys offen

Craft them and make your profile look sexy

I just use em to craft a bunch off badges :)

Weigh them up. See what cards you have for each game. The sets where you have no doubles you keep to make badges. The ones where you do have 2 of a kind you just sell all of them,. That way you'll make the wallet money to buy the cards you need to complete the others sets. Alternatively, you just sell them all and make wallet money for buying games.

I'm a collector so I like to keep them and craft badges.

If you want more games for them visit one of these guys. I got like 80% of my games from them.
http://www.steamgifts.com/trade/nmRfX/
http://www.steamgifts.com/trade/OJiA1/
http://www.steamgifts.com/trade/WBl5g/

I change cards for games. Have Borderlands 2, Serious Sam 3 and STALKER Call of Pripyat :)

I'd say trade it. You get much more value out of your cards by trading them for games than simply selling them off.

If you don't care about badges and steam level then I'd say just trade them for games. It's better trading them then selling them on steam market because you wont get a lot of money for them.

Some people trade their bundle leftover games for anywhere from 5 to 15 cards. I've got a lot of games that way.

Check SteamTrades and Barter.vg for trades.

Sorry about stalking your profile ;) , but thanks to that I know you have Steam-Level 3. Thanks to that some suggestions/food-for-thoughts:

• finish that Community Badge for one free Steam-Level (it should be doable without spending any kind of money)
• during previous Big Sales one of things you could do was voting for next game-on-sale and that voting was giving you free Sale-Cards, which you could sell for few cents; but to get said Sale-Cards, you had to have high-enough Steam-Level, during Xmas Sale you had to have level 8; Sale-Cards sell for at least 10 cents each, which means you could get around $1 each sale - maybe it might be worth investing in some levels to get it?

Donate them to homeless people.

if you use the site "tremorgames" you can trade the cards in, and get points out of them, from the points you can buy games or something else. (I think O_o)

50 cards is not very many but trade them for games here is my link http://www.steamgifts.com/trade/htb4N/h-lots-of-games-w-other-games-i-dont-have

Wenn dir das Craften wirklich total egal ist, dann verkaufe die Karten beim Summer Sale, so wie die Leute hier schon gesagt haben :)

Sell them.