It’s pretty simple. You decide what you want to sell, tell it how much posting it will cost, have a look at each site to see how much they are going for, and it will calculate all the fees and tell you how much profit you expect to make on each site. It has to make some assumptions, but it should give you a good idea.
Currently it only has data for CDs, DVDs and Blu-Rays, but I may add more shortly (let me know if there are any requests).
Of course there may be bugs, so let me know your comments and suggestions below. I accept no responsibility for any errors etc.