I'm of the opinion that there are NO niches that aren't filled. With 180 million active listings, there's very few parts of the ecosystem that don't have at least a few established sellers.
That said, it doesn't mean you can't sell and make money. Just that you probably won't be the only one selling any given object.
Try to find closeouts where you can purchase multiples of the same items. That way you can sell the same thing over and over. One of a kind items ( ooak ) are tough, they require LOTS of time taking pictures, editing, writing ads, etc.
I was lucky to picture and list 30 OOAK items a day.
For example, I would go to Big Lots and other overstock liquidators and buy 30 packages of boys Power Rangers underwear for $2 a set, and sell them for $6.99 + shipping.
Even if they were different sizes, the pictures would be the same, and all you have to change is the title and item specifics regarding size. If they sold well I'd scour all the stores in town and buy the entire inventory in the city. This multiplies your listing power by a factor of 10-20X.
Seasonal changes when they mark things down to $1 is good too. Once again look for multiples.
If there's anything in your life where you say "I get a great deal on this widget and everybody else pays too much", that's the kind of thing you need to look for.
If you're in doubt about something, buy one, list it immediately, and if it sells go back and get the rest. If it doesn't you can return it after a few tries, but you've got to turn the product fast.
Dealing in large price items brings large rewards, but also large risks. Can you stand the thought of losing a $100 item? Can you stand the loss of the money? Because, as we can all tell you, you ARE going to lose your money and your item both at some point, and it's going to HURT!
If you make $10 a pop instead, the risk is smaller, so is the profit and the possible loss.
Can you mail 25 items a week, or 50, or 100? Can you source that much inventory? Can you deal with the BS EB and PP is going to put you through?
Maybe you have a skill you can sell instead of an object. Maybe you're a crafter who makes embroidered pillows or eskimo hats or custom wedding/anniversary cards or personalized cellphone cases. Perhaps you can add value to an existing product and make it unique and desirable.
Be creative.
Good luck!
Finding a Niche 
