use an auction site that lets you pick a 'reserve' price, where you are not obligated to sell the item if the reserve price is not met. This way, you can calculate what you would receive after said site takes their cut, and have the freedom to say no if you don't get what you want (while still having the choice to compromise on the price if you can actually live with the highest bid minus the site's cut).
Say... you really want to take home roughly $300 and the site takes 20%. Set your reserve price to $380. If it's met, you'll get at least $304 in your pocket.
Can't think of a better answer than that.
Edit: and don't forget to specify how much you will add for shipping, or include that in the price and specify where you are willing to ship it to....