No cost but a donation if it sells

Does this sound reasonable to you all. I will happily sell items on TAG for no cost but if the item sells, perhaps a small donation could be made which will go at the moment to the Domaine du Sautou, near Castanet for the benefit of the children of Mali. Every 10 or 20 euros helps. Obviously when I sell a house maybe a larger donation could be given and it still applies that a 30 euro donation is needed for the 6 months display. . Notice, I say when rather than if,  after, cars, cranes, antiquarian books , dogs, cats,  why not a house?