Problem:
I am having trouble understanding the supply and demand relationship between supermarkets and their supply of shopping trolleys to customers, and the customers demand for the trolleys. At first glance it appears that no market exist because trolleys are given to customers for free, but this can't really be the case. For example the, provision of larger numbers of trolleys becomes increasingly expensive to supermarkets due to increased storage, collection, maintenance costs etc, giving an upward sloping supply curve. So how is the "price" of shopping trolleys signalled for customers and supermarkets?