what kind of things are you selling?
I supose prices vary in differnet parts of the country, but since you want to get rid of stuff, price it cheap. Most things $1.00 or less. Unless it's something really special clothes should be .25 or .50 cents, same with books. CD's 1-2 dollars. Nick-nacks and odds and ends .25 -- $1.00. Small apliances like coffee makes and such could be a few dollars if they are in good condition, particulary if theres' an outlet avialable so people can test them out. Kids toys should be really cheap too. (unless you're selling legos or complete sets of playmobils or other "name brand" toys -- those can be a more)
Bring lots of coins and one dollar bills with you for making change. And have fun!!