Let's say the shop sells once in a lifetime items. You won't buy again likely. So I can't setup a discount for a customer A that bought an item already and then recommends that item/shop to another person B that then buys an item from the shop. Also I don't want to discount possible customer B as this wouldn't benefit customer A. To still stimulate the behaviour I would need to discount/refund customer A's already placed order if customer B buys an item.
What would be the best way to do this if there is any?