I also assume you're new to PC, so if you are, you can get amazing deals on games via bundle sites, such as Humble Bundle, Fanatical, GreenManGaming, and etc as they sell game keys you can redeem via Epic, Steam, or etc... Most game keys are for Steam. Can use tool site to help find best deal on games isthereanydeal.com, please note games do go on sale, so don't be scared off by games full price tag as those are... Source: almost 3 years ago

For cheap games, depends what games you want to look for, action, fighting, racing, what you looking for? There's also bundle sites that sell bunch of games for low price which are mostly sold Steam keys, which you can redeem on Steam. Such as humble bundle, fanatical, indie gala, or etc... Can use site like isthereanydeal.com to look for certain game deals as well. Source: about 3 years ago

I think I used steam in conjunction with https://steaminventoryhelper.com/. Source: over 1 year ago

Only thing I can think of would be https://steaminventoryhelper.com/ But I don't think it's open source :(. Source: almost 2 years ago