Sorry, going to have to disagree here - this goes back to my statement on opening a restaurant. We have all the demand in the world for fatties that want to go out to eat every-day. It could be fast food, sit down restaurants, coffee, etc... we have a HUGE amount of people that demand restaurants of all shapes and sizes. Again, you will be crushed instantly, ESPECIALLY if you don't have any upfront capital (which most individual people do not).
If it's not a niche product, that means there is already a producer for it and there is already someone making the product. What does that mean? It means someone else has already established the market, and has likely already fine tuned it with cheap manufacturing and cheap materials than you are likely to need to buy for much more expensive.
Of course, it all depends on what your goals are though. If you want a thriving business, you better have a niche. If you want some side income, go for it. They are two very different things.