I don't have anything to add to all the wonderful advice you're getting, but I have a rule for my garage sales - it never comes back in the house. At the end of the sale, I load whatever didn't sell goes into my truck and straight to Good Will, the second-hand shop, whatever. Also, if you're on Facebook, check to see if there's a buy-sell-trade page for your area. You can ask to join the group and advertise your garage sale on there. It's also a good place to advertise anything that's more high dollar than the rest of your stuff.
have fun!