I found this site when trying to figure out the Ebay and Paypal Fee's before posting any product on the site. It may or may not be to any use for most but I found it very pleasing.
The way I do it, Is I compute the fee's on the site first, after I sell the item I go back and check the actual fees. So far It's dead on. It's probably not 100% accurate but it's a good way to get an estimate on what your fee's will be. Everyone has their own method, this one is mine.