Online selling isn’t what is used to be. To be successful nowadays, you either need a large budget or a small niche.
There are a few online research tools to help make your job of finding a niche easier.
The Market Research Wizard: This will cut your search time down significantly. You also get pretty stats like this:

Overture: You can use Yahoo’s data of monthly search terms to find out what people really want to buy online.
Above all, I recommend finding a product or industry you are familiar with. This will allow you to understand your customers better so you can tailor your auctions to their needs.

