Garage sales are so 1994. If your parents have a lifetime's worth of old toys, gently used furniture or outdated electronics piled up in the attic, grab an industrial-strength garbage bag, toss out the true junk and sell the rest online. Twenty-something millionaire Cameron Johnson was only 12 years old when he made $50,000 selling his sister's Beanie Baby collection [source: Miley]. He had her permission, of course.
After you've cleaned out the attic, you could also make the rounds at local garage sales, thrift stores and junk yards looking for undervalued merchandise that can be sold for a profit online.
Sites like Craigslist and eBay are ideal for converting all that trash into someone else's treasure. It's also a great way to learn valuable marketing skills like "How to take a blurry, low-light picture of a grungy old desk to make it look brand-new."
Along with that picture, you'll need to write an appealing description of the item for sale; check other site listings to get an idea of what others are charging for the same merchandise. If you have to meet the buyer in person to hand over the item, be sure a parent is present, so you don't get ripped off.