If free markets existed, that is how they would operate. Sellers would be small, independent, have perfect knowledge of the market and would always seek to maximise profit.
But free markets don't exist. There are many reasons why merchants might not always aim to maximise profit. They might instead seek the security of a loyal customer base, for example. In many markets, sellers ruthlessly compete on price, often at the cost of short term profit. Supermarkets in the UK is a good example. If transaction costs for supermarkets were reduced that would absolutely be seen by the customers.
But free markets don't exist. There are many reasons why merchants might not always aim to maximise profit. They might instead seek the security of a loyal customer base, for example. In many markets, sellers ruthlessly compete on price, often at the cost of short term profit. Supermarkets in the UK is a good example. If transaction costs for supermarkets were reduced that would absolutely be seen by the customers.