I have a store and have been considering inviting some designer friends to do an event here where we have a one day sale, drinks, and snacks and a few people out on the sidewalk. I once did this with a friend who had a store and we all kept whatever we sold; I have also cleared entire racks for designers where they are expected to give me a percentage of their sales. (I am so casual in these arrangements, I usually agree to 20 % and then actually don't end up taking anything, even if I spend money on all the refreshments on my own.) What is the norm with this sort of pop up arrangement?