Fudjj is right, a great way to find niche markets and hot products to sell is by selling stuff from around your home.
There are two excellent reasons for starting out in this way:
(1) Feedback is hugely important for any eBay seller. With a feedback rating below 10, it is going to be difficult to get people to take you seriously. The first 100 sales are the hardest as buyers look for high feedback as reassurance that they can trust the seller. Generally, the higher your feedback, the better - with powerseller status being the ultimate goal.
(2) You get to sell a range of products and see what works well. The results may surprise you!
Nothing beats knowledge gained from experience and selling stuff from around the home is the best way to get it without any risk. After some practice with goods from around the home, I recommend buying small wholesale lots of products that most people use on a daily basis.