In my state a person is allowed to sell x amount of automobiles in a year before they have to get a car dealers licence. It could be like that. A person could be allowed to make a little money on the side up to a point. Then you have to get registered & start paying taxes. Ever seen the show Firefly? It's in the future & prostitution is legal. They're called "Companions". The best companions are educated at special schools and are sought after by elites across the galaxy.