Well that's theeeeeeeeeeeee question isn't it?
I'm new at this myself, just about a year now, but one thing I've learnt is sell what you're interested in. If what you're interested in is saturated, think around it. There must be someway into the market via what you already know about. I say what you know, because you're gonna have to answer questions & write listings and if you don't know what you're on about......Plus if you know what to look for in an item, then you're ahead of the game already...
Once you figure that, you need to find yourself a good supplier!
If it were easy, the world and his wife would be doing it....