eBay has some good tools that can help you find out what is popular.
First, take a look at the trends report in the Seller Central area. There is an updated PDF document that lists what is selling well in each category published monthly.
To get some ideas for what to sell, start reading newspapers and magazines. You'll soon start to spot emerging trends and items that maintain consistent popularity.
Then use eBay's completed listings search to check out the sell-through rate. Ideally you want products with a sell through rate of 70% or more.