Ask your self how much you want a year. Lets say 50k a year for your self = Z. So then ask your self how many sales are expected on the site within a given ye

Ask your self how much you want a year. Lets say 50k a year for your self = Z. So then ask your self how many sales are expected on the site within a given year = X. And Y is your amount you should charge per item. z/x = y. Shoot just a little low unless they are very happy with the idea. Tell them that over a certain period of time, if X exceeds your personal number, you will reduce your costs... Seems fair and if they sell more, you get more... Each party wins.