Discs are the best way of making potential recruits stay in the game, since practice is what new players need most. The more discs there are around, the more likely people are to do frisbee related things in their own time, and the more likely they are to stay in the club. You will probably find that you don't sell many discs in the first week, but be prepared after that. People want discs even more after they discover the joys of Ultimate. Glow in the Dark and Ultra Violet discs sell well.
Discs are also good to have stuck up around the stall.
Coat hangers can be bent into a shape that will happily hold and display a disc. T-shirts are often good for advertising, particularly if the design is good. As a seller, people are unlikely to want to pay much, so they don't generally go quickly. Other Important Stuff Money tin Preferably a metal lockable one, but an ice-cream container will do. Remember to get plenty of change from the bank beforehand.