Easy for who? Amazon knows which seller sent in the inventory for every order, so they know to punish the right seller when a complaint comes in.
You are correct that customers could themselves choose better sellers, but most customers don't care and prefer cheaper over more reliable. That effect is not enough to help "enormously".
You are correct that customers could themselves choose better sellers, but most customers don't care and prefer cheaper over more reliable. That effect is not enough to help "enormously".