A: At the dollar store, there is always somebody who has to buy something even if they don't need it.
B: Yes, yes! That's me.
I am told that 気が済まない means "won't be satisfied unless," or when one "must (do something)."
If I translate the expression literally I get confused--
済まない finish; come to an end; excusable; need not
Would anyone be kind enough to give me a clear and thorough explanation of this expression? I think my main confusion is as to which verb 済まない comes from.