Keep your costs down. In order to turn a profit on what you're selling, choose to sell an item that's inexpensive to make. Lemonade is a classic because all of its ingredients (water, sugar, lemons or lemon juice concentrate, and ice) are fairly cheap and easy to obtain. Popsicles are another favorite because they can be bought in large quantities from the store!
The stuff you can't sell online, you could sell from your garage on the weekends. Many neighborhoods plan annual or bi-annual yard sales. If you have items to sell, this is a great time to do it as the neighborhood as a whole can bring in a lot of traffic and help you perform better than you would on your own. If that's not possible, consider partnering up with a couple of families in a popular neighborhood.