eBay Pulse at http://pulse.ebay.com allows you to see the most popular searches, largest stories and most watched items.
You can view the same results by category as well:
This information can help you decide what items to sell on eBay or if you’re an eBay affiliate, it may help you shape the content you create. It can even help you come up with info product ideas (ex. “how to sell XX on eBay”, “how to find cheap XX on eBay”). It’s a huge marketplace and the opportunities are endless.