Complex sentence: Reputedly he enjoyed the experience so much that he became a grifter and moved to Las Vegas to work the marks there, because "There are more marks per square mile in Las Vegas than anywhere else in the world".
Split into simpler sentences: Reputedly he enjoyed the experience so much that he became a grifter and moved to Las Vegas to work the marks there. He went there because as he puts it "There are more marks per square mile in Las Vegas than anywhere else in the world".