It's been a number of years since I was involved in the e-commerce side of work here but we watched a lot of small web based business start up and fade away in a year or two. The ones that are still running are the ones that found a niche market. Hard to find stuff.
If it's mainstream, you probably can't compete with the pricing of the 'big boys' so you need something that is unusual enough that you will easily grow to be one of the top Googled pages for your product but yet also in demand enough to make money.
Honestly, weather you jump on a pre-existing brand in one of those "get rich with us" companies or start your own business, the success rate is very low.
Don't want to discourage you though. If you make smart decisions, there is an easy $ to be made.
PLus my info is a few years old. The online market could be very different now so take my advice with a grain of sea salt.
