I came across this eBay tool for researching sales by category. You can see Top Keywords, Top Bidding items, Top Sellers, Hot Keywords.
http://labs.ebay.com/Catman/
When using the tool, it may appear to not work. For example, on Top Keywords, you need to select a date on the top right. You have arrow keys to change the date.
Limited usefulness in my opinion but it might be of interest to someone.