How does selling and buying junk work?

I jsut have no idea wat junk is how do people make money from it ect... What is this?

 

Making money from junk is like you merch stuff till it starts to crash, then swap it to less junk stuff, and then sell or swap for non junk stuff Also, buying junk from ge lowest and selling it for med (example; Lending items) will get you nice 5% profit

 

Junk is just to sell items for more than they are worth. Like rares.

That is how I viewed junk as anyway.