I need help with this Because i see people do it and i dont see how to

You can't sell games but you can refund games if you have played less then 20 hours and you've had it for less then 2 weeks I think

https://support.steampowered.com/kb_...6695-QIKM-7966

There's the tutorial.

You could also have games that are meant as gifts and sell those, that's what I've been seeing.