Sorry it's not very attractive, yet. I've written my own database (in Excel) to manage my card collection. One of the cool features I added was the ability to generate webpages of cards I need and those pesky duplicates. You will find both lists here, grouped by sets and individual players.
I have not completed entering the pricing information, but wanted to get the cards on-line. My general rule of thumb is that I will trade Beckett Hi for Beckett Hi, sell at 40% of Beckett Hi, and buy at 25% of Beckett Hi.
After I finish updating the pricing (and take care of that big box of cards in my room that has been waiting for the program to be finished so they can be filed away), I plan to work on improving the navigation. Check back often, as I believe this will be a nice site in the very near future.
Without further ado, here's the list of what I'm searching for and trying to get rid of :)