Books- individually books may not be worth very much. I heard of one story from a woman that bought a couple hundred books at a yard sale for $5. She then sold each book on Ebay for a dollar or two, she made a tremendous profit.
Toys- With the holiday season here I thought I would highlight another niche. Every single year there is a hot toy for sale around the holidays that sells out in stores. People then resort to Ebay to buy these gifts even though they have to pay double the price. This means that if you can predict which toys will be popular, you will be able to double your money.
Antiquities- Anything old someone might think is worthless, but to you, you see an ebay opportunity. Even if something old seems worthless to you put it on Ebay. There is a very large market looking for anything old and possibly valuable to buy.
There are many other niches out there, these are just a few. All you need to do is find the right niche for you and you could start you Ebay career.