I recently did a search for can openers following up on someones post and noticed that many of them were being sold each day. Its really hard to say. Just search something you are interested in selling and see how rare or popular it is. and how cheap you can aquire said items.
There are many areas still barely touched on ebay. I bet the items you see the less auctions on will be the most profitable niches for an ebayer willing to jump into that market.
One sellers jumped in and sold anything related to rubber ducks. Now she sells over 1300 items a month related to rubber ducks.