Here are a few links which are very useful:

1，http://pulse.ebay.com/
The link above provides the lists of popular searches,largest stores, most watched items.

Another link:
http://keyword-index.ebay.com/keyword-index.html
This link lists the hot keywords on eBay.

If your item’s title includes one or more hot keywords, when the viewer searches on eBay with those keywords, your item can have a chance to show up.

2，http://product-index.ebay.com/best_selling_1.html
The link above lists the popular products on eBay. If you sell those products too, you may be a loser since the competition is very severe. But if you sell their accessories, it may be easy to succeed.

3，http://pages.ebay.com/wantitnow/
The link above lists the itmes that eBay customers want to buy but can’t find.