Nice one dale, time for me to throw some ideas into the mix.
Item Trackers - You could put in any item or a General Phrase, and the tool will monitor it and tell you if and when you have been undercut
Seller Top Sales Report - See the best selling items per seller, prices,sale dates.
Keyword Suggester - Start Typing in a item and based on what people search for, it tells you the best words to use in the Title.
Cheap Checker - Show the cheapest an item has been sold for and who is selling it. Type in what your selling, it analyses the results and tells you who the cheapest is at the moment.
Inventory Reporting - Ability to import your inventory and run a report against your items compared to the market and get a score rating on each on how well your items are, your price, etc etc.
Perhaps even multi platform research, not just eBay, but something like Amazon as well to cover a wider market.
OK, there are a few ideas. So come on, lets get a dialogue going on this so the development team knows what the members really need.