An exotic setting or technology is usually the first thing that crosses my mind when I have a new story idea, which I populate with characters after the fact. But today a character popped into my head and the setting followed afterward.
Elmer Quincy inhabits a future world where interstellar commerce is common, as is contact with alien races. Under the principle of "one man's trash is another man's treasure," Quincy makes his living collecting biological waste and trash from certain alien cultures and trading them to others.
Dirty in every sense of the word (physically I imagine him obese, in stained and greasy overalls, with a stubbly face; in business I imagine him a tough haggler and quick to make a profit even when not strictly legal), he distinguishes himself by what he refuses to do: join a criminal syndicate involved in alien slave smuggling. Disliked by the law, pursued by criminals trying to kill him, perhaps befriended by an odd religious sect, Quincy becomes an unlikely hero and slave liberator.
I really like this story idea, but as my queue of stories I want to write is already rather long, this one is up for grabs...