eBay Research

This research tool is highly recommended and used by many eBay Power Sellers, to gain an advantage over their competition and to make money selling on eBay.

It's called Terapeak.

Features :
Research your competition easily
Discover new products that make you money using eBay data
Get eBay seller research sales reports
Research when to sell, how much to sell for, and what keywords drive profits

More information at Terapeak