From what I know there is at least 3 ways of making money, easiest one is to kill members of the opposite faction, rip their patch off (equip your knife and press space) and sell those to a trader (I'm a loner and I get 5000 per bandit patch).
You can also chase down mutants and gut them, you'll get parts that you can sell to a trader. I got 650 for a flesh eye, but if you manage to kill more dangerous mutants (bloodsuckers, controllers, etc) you'll probably earn more per parts.

Or you can try to find artifacts, you'll need a detector, some batteries and maybe some other stuff, I don't know how it works I didn't get that far.

Thank you man really appreciate your answer ! :)

How can you sell stuff I don't see the sell option anymore