Unless the market output goes to buyers who have the highest valuation of the product then there is an allocation inefficiency. An equivalent requirement is that each buyers valuation of one more unit of the product (marginal value) is the same and higher than the valuation of any agent who has not bought the product.

If there is an allocation inefficiency then conceivably a buyer with a low valuation for the last unit he purchased could sell that unit to a user who has a high valuation of the last unit she purchased and they could both be better off. If there is some way of re-allocating among buyers the total market output, such that the consumer surplus is increased, then there is an allocation inefficiency