Well, you are going to have small issues either way you go... If you charge you may not get very many people to buy your balloons, but if you do only tips you may find yourself giving away a lot of balloons because the people at the market think you are getting paid by the farmer's market to be there. Personally, I would charge a few bucks a balloon and have a small amount of balloons that you have ready for those who can't pay or refuse to-(swords and dogs...)
Congrats on getting that gig though

Jake