Similar to selling on Amazon, eBay is a place to make money selling just about anything you can think of as a side business idea, with the added excitement of using the auction selling model to sometimes get way more than you expected for that antique baseball card you picked up at a local street fair. To turn your eBay selling into a legitimate business idea, look for opportunities where you can buy in-demand products at a discount—and later sell them for a profit. Eventually, you can even take the lessons learned from your eBay store and build a full on blog or eCommerce business like what the team over at Gear Hungry has done.
Anyone can sell goods on Amazon, provided you have products to sell (or buy low, then resell). If you’re the type to hit all the local garage sales each weekend, there’s all sorts of valuable things that can be resold online as a side business idea. If you want to step your Amazon selling game up, check out this detailed guide to Amazon and eBay retail arbitrage on Entrepreneur featuring an interview with Julie Becker and several drop-shippers who've grown this home based business idea from side business idea into a lucrative money-maker.
As for other tips, all I can say is “a penny saved is a penny earned”. If you are running out of ideas to make money, start saving. Some quick “low hanging fruit” tips to save some quick cash include – switch to MetroPCS or Boost for cellphone (I only pay $20/month), switch auto insurance to Insurance Panda ($25/month), and start using GasBuddy (saves me like $100/month at least. I drive a lot).