You're not the 'efficient seller' if you lose money at it.
Though inefficiencies reduce profitability, the inference that negative profitability implies inefficiency is invalid.
Let's unpack your own reasoning here: An inefficient business will be unprofitable. Amazon is unprofitable. Therefore, Amazon is inefficient. If A, then B. B, therefore A. The category of error you have made is termed "affirming the consequent", colloquially known as Modus Morons.
Profit is, to quote WP, "the difference between the purchase and the component costs of delivered goods and/or services and any operating or other expenses." Therefore negative profitability could result from either inordinately low pricing or inordinately high expenses, or both.