After you have seen the dueling seen you can duel for other people's beans. The way you can duel is on some free time, go down to the Entrance Hall, if you are coming in from outside than go to the door on the right but if you are coming down the stairs than go to the door on the left. Pretend you are going to the Great Hall and stop right in front of the door. Turn to your left and you will see a boy standing there. Go and talk to him and he will offer to duel with you fo a certain amount of beans. You will work your way up to the best person. The easy way to beat them is to let them shoot a spell at you then move before it hits you about five times. Then shoot rictusempra at them. They might use expelliarmus to defend them selfs that means you have to move. If you get five-hundred beans you can go and buy Quidditch armor from George. Then you can earn more beans again.