Marc could not have said it better. You first need to find out what you want to sell then creating a market study based on what is currently the trend on eBay or on any other eCommerce sites.
The key point to remember is that the internet is in constant flux. You need to work hard to keep tabs on current trends to make maximum profit. If Craigslist is doing well, sell on Craigslist. If you find a niche on eBay, sell on eBay.
Many sellers sell on 3 or 4 different sites to get the benefit of numerous audiences! Keep experimenting, testing and reading the forums and you'll do just fine!