{Buying} Steam keys bunch of them

https://www.steamtrades.com/
https://www.humblebundle.com/
https://www.indiegala.com/
https://www.bundlestars.com/en/
https://groupees.com/
http://otakumaker.com/
https://barter.vg/

If you're only looking for the cheapest games with trading cards or for the +1 to your Steam library, and don't care about playing them, there are hundreds of different ones available at http://dailyindiegame.com/ and http://steamground.com/en/wholesale starting around 5 cents...

Steamground seems to be around 100% cheap greenlight things, but Dailyindiegame even has some ok older games that slipped in somehow, so good luck! :)

I usually use this site to go to other sites to catch the best deal https://isthereanydeal.com/