N_dissatisfied > max(0, N_for_sale - N_individuals_and_couples_buying)
When N_for_sale > N_individuals_and_couples_buying, it is still possible for N_dissatisfied to be > 0 for the reason you give. But N_dissatisfied must be > 0 whenever N_for_sale < N_individuals_and_couples_buying, even if everyone is limited to having at most one.