You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
eBay is the most popular global auction site around and is a great place to sell items you no longer want. If you are looking to earn some extra cash, start out by looking around your house for things you no longer want; list them on eBay and watch as customers bid on them. You can also purchase clearance sale items and thrift store finds and make a couple dollars profit by listing them as well. The key to selling on eBay is to do your research and find out what the hot items are.