good tips/points by everyone..just remember now that our country and most states are broke, Uncle Sugar wants his cut and the TAX people are watching the internet like hawks...so any advertising of selling items now makes you a business and liable for sales taxes as a business. Craft shows will see lots more paper work this year as the local tax people hit them with forms for vendors. OK now makes you pay sales tax on your yard sale!
So before you head into ANY form of selling be sure you do your homework for what you are liable for in your city/state as far as taxes go!