The toy model: The poor person rents an apartment that he can barely afford. He discovers that he can now sell his kidney, but so does his landlord. Suddenly his landlord raises everyone’s rent by the price of a kidney, so they all sell their kidneys to pay the increased rent. He and everyone in his situation are now worse off than before; only the landlord and the kidney buyers are better off.
The toy model: The poor person rents an apartment that he can barely afford. He discovers that he can now sell his kidney, but so does his landlord. Suddenly his landlord raises everyone’s rent by the price of a kidney, so they all sell their kidneys to pay the increased rent. He and everyone in his situation are now worse off than before; only the landlord and the kidney buyers are better off.