I had a kid in my dorms at college who was a bookie. He wasn't too big but still cleared about $500 a week. Pretty good for a full time student.
I think clients were mostly word of mouth. You start small and branch out thru references.
As for getting stiffed, my guy was a cash only player. No cash..no play.
His "guy" was in Brockton Mass and it was like the Whitey Bulger family reunion when I dropped off my betting slip one day. Even Swole would be nervous.