Here are Tips from Aaron LePedis, the Garage Sale Millionaire.  Yes, this guy became a millionaire flipping Garage Sales finds.  He offers a few great tips:
1. A garage sale is a great way to get rid of stuff, meet people and make money!
2. Make sure to get people talking.  Don’t price items below $15.  Let them ask How much is this? Then, reply How much are you willing to pay for it?
3. Only price items over $15-$20.  Otherwise, they will offer less then what you want for this item.
4.Put A LOT of signs.  LePedis says that not enough signs is the number one mistake. Also, tell Facebook friends and use the app TagSellIt.
5. If you are thinking a flipping items, avoid big items.  They are hard to sell.
6. Bring your cell phone and research on the spot how much an item is really worth. Use that information to bargain.
7. Be willing to fix an item to get a little more for it.