Don't be bullied by the people who want to haggle the price down to nothing. Don't be afraid to say, "No, I want this amount for this, come back <what ever time you are closing> , I may consider it then." It's amazing how brazen some people can be at a yard sale, even if it's for charity.
I helped out at a fundraising yard sale for a quilting group. We had some really nice things, some fabrics that had been donated that didnt' work for quilting, nice clothes etc.
My friends sent me home because after about 2 hours of people saying, "this isn't worth that amount, I'll give you .50 for this entire box" I finally started telling the rude people, "if you don't like the price, go away. No one is forcing you to buy from us. We are raising money for charity."
You could do a raffle also, do you have a baby quilt finished or something small yet cute already finished? You can sale tickets for $1. Have the drawing at the end of the yard sale.