A lot depends on how much of your income you are looking to get from eBay and how much work you want to put in because it can take over your life more than working for someone else if you let it.
Back in 2003, we decided to homeschool for a year to work ADHD meds out of our 2 boys' system without the stress involved in the public education classroom. So I quit teaching for a year and then began substitute teaching when they went back to public school in case there were problems. This school year is my first back in my own classroom and it has been fun.
We were eBay buyers and occasional sellers at the time so I began checking out things over the summer to see if there was a way to supplement our income (since I wouldn't be working) without having any extra money to do it. It took me about 2 months of research to figure out if the things I knew I had, that would be renewable resources, would sell consistently. At the time, my mom had a huge flower bed that had not been thinned for at least 15 years and wasn't blooming well because of that. I also had a couple of breed of chickens that I had hatched eggs from and after watching those, I knew that some would sell there to help pay for animal feed. As I was a teacher and very frugal, I also knew how to create neat classroom items and find supplies for cheap.
To make a long story short, I was able to begin operating an eBay sales business using the things that I knew and I had on hand, we were able to utilize the whole sales/shipping/recordkeeping process as part of our homeschool curriculum, and I didn't have massive stress trying to locate/purchase things and then worry about whether they would sell. If my stuff didn't sell, I was only out the auction fees and I have found over the last 4 years that if it isn't selling right now, wait because it might sell better in a month or so. I was able to make something like $500-$600 each month without leaving my house, the kids learned many business skills, and I have developed a sideline that paid for all their school activities, small purchases, and just let us have fun without stressing. If I need to take a break I just don't list for a while. While finishing my Master's degree, I took a school year off to keep from being too stressed, but begin listing when I finished and things took off.
The biggest key (as others have said) is to remember that you need to research your sales in depth and keep you overhead down so that you don't have to take a hit on loss. Someone mention handcrafted items which isn't a bad idea. But if you have too much time involved for not enough money, it might be easier to work for someone else and let them have all the bookkeeping and overhead worries, because it could sap the joy out of making the items when you have to rush to finish in order to ship. BTDT
~Lisa~