Well for one thing, you have to have an idea on what you wanna sell, and whether people would be willing to buy or not. If it's not a rare product, you have to think 'Why would people want to buy from me, an unknown business, when they can get it from a place they know and trust already?' And if it is rare, what's so good about your product? Why should I buy, instead of saving the money for gas or for other important priorities? These things take a lot of thought.
Not to mention, many, many, MANY people aren't willing to hang in there when times get tough. You need drive, determination and discipline for things like that. How many people you know make New Years Resolutions and actually stick with them?

So I could see why people aren't too willing to do it.