This is a little tool I built for when I sell on Reverb, you enter the total price you are selling an item for (including shipping) and you will be shown the price of the fees from paypal, reverb direct checkout and paypal then the total you would get if for example you were selling using paypal or direct checkout. If you are offering Free Shipping (which i usualy do) then you can also input the shipping price (which fees are not included on) and it will be deducted just to give you an idea of what you'll end up with. Obviously there's no warraty here if the program gets the wrong price, this is just meant to help get an idea of the money you loose from fees specially when people are giving you offers.
PS. I have no affiliation with Reverb, their name is their trademark.